Skip navigation

Treat dads & grads: buy gift cards online & earn a $25 or $50 promo card! Gift Cards & Restrictions

Sign In

  • All Clearance
  • Clearance Under $25

  • New Markdowns

  • Women
  • Men
  • Kids
  • Shoes
  • Bags & Accessories
  • Home
  • Beauty
  • Women

  • Clothing
  • Shoes
  • Handbags
  • Accessories
  • Jewelry
  • Beauty
  • Men

  • Clothing
  • Shoes
  • Accessories
  • Kids

  • Girls' Clothing
  • Girls' Shoes
  • Boys' Clothing
  • Boys' Shoes
  • Baby
  • Bags & Accessories

  • Shoes

  • Designer

  • Home

  • Beauty

Shop Now
  • All New & Now
  • Father's Day Gifts

  • Summer Style Essentials

  • New From Our Top Brands

  • Top 100 Deals

  • Just Landed from Nordstrom

  • Linen Shop

  • Vacation & Resort

  • Women

  • Clothing
  • Shoes
  • Handbags
  • Jewelry & Watches
  • Accessories
  • Men

  • Clothing
  • Shoes
  • Accessories
  • Kids

  • Girls
  • Boys
  • Baby
  • Home

  • Beauty

  • Trending

  • Women's Bold Color
  • Women's Preppy Style
  • Women's Ruffle & Lace Details
  • Men's Baggy Jeans
  • Men's Preppy Style
  • Jelly Shoes
  • Beaded Accessories
  • Coastal Chic Accessories
  • Soccer Style
  • Flash Events

  • Marc Jacobs Up to 50% Off
  • Diane von Furstenberg Up to 60% Off
Father's Day Gifts
  • All Women
  • New Arrivals

  • Best Sellers

  • Summer Essentials

  • Linen Shop

  • Trending: Preppy Style

  • Trending: Ruffle & Lace Details

  • Trending: Bold Colors

  • Nordstrom Made

  • Clearance

  • Clothing

  • Activewear
  • Coats & Jackets
  • Dresses
  • Jeans & Denim
  • Jumpsuits & Rompers
  • Lingerie, Hosiery & Shapewear
  • Matching Sets
  • Pajamas & Loungewear
  • Pants & Leggings
  • Shorts
  • Skirts
  • Suits & Separates
  • Sweaters
  • Sweatshirts & Hoodies
  • Swimsuits & Cover-Ups
  • Tops
  • Plus-Size Clothing
  • Shoes

  • Athletic & Sneakers
  • Boots & Booties
  • Comfort
  • Flats
  • Heels
  • Loafers
  • Oxfords
  • Sandals
  • Slippers
  • Handbags

  • Sunglasses & Eyewear

  • Jewelry & Watches

  • Accessories

  • Beauty

  • Shop by Occasion

  • Concert & Festival
  • Night Out
  • Vacation & Resort
  • Wear to Work
  • Wedding Guest
  • Designer

  • Contemporary

  • Young Adult

Shop Now
  • All Men
  • Father's Day Gifts

  • New Arrivals

  • Best Sellers

  • Linen Shop

  • Golf Shop

  • Summer Essentials

  • Sunglasses Under $100

  • Trending: Baggy Jeans

  • Trending: Preppy Style

  • Clearance

  • Clothing

  • Activewear
  • Blazers & Sport Coats
  • Coats & Jackets
  • Dress Shirts
  • Jeans & Denim
  • Joggers & Sweatpants
  • Lounge, Pajamas & Robes
  • Pants
  • Polo Shirts
  • Shirts
  • Shorts
  • Suits & Separates
  • Sweaters
  • Sweatshirts & Hoodies
  • Swimwear & Board Shorts
  • T-Shirts
  • Underwear & Socks
  • Shoes

  • Athletic & Sneakers
  • Boots
  • Comfort
  • Designer
  • Dress Shoes
  • Loafers & Slip-Ons
  • Oxfords & Derbys
  • Sandals & Flip-Flops
  • Slippers
  • Accessories

  • Bags & Backpacks
  • Belts & Suspenders
  • Hats
  • Sunglasses & Eyewear
  • Ties & Pocket Squares
  • Watches
  • Dresswear Shop

  • Contemporary

  • Designer

  • Young Adult

  • Big & Tall

  • Grooming & Cologne

  • Shop by Occasion

  • Concert & Festival Outfits
  • Vacation & Resort
Shop Vacation
  • All Kids
  • Back to School: Kids

  • New Arrivals

  • Best Sellers

  • Swim Shop

  • Kids' Matching Sets

  • Kids' Value Packs

  • Summer Essentials

  • Sneakers Under $25

  • Clearance

  • Girls' Clothing

  • Toddler Girls (Sizes 2T-4T)
  • Little Girls (Sizes 4-6x)
  • Girls & Tweens (Sizes 7-20)
  • Girls' Accessories

  • Boys' Clothing

  • Boys (Sizes 4-7)
  • Boys (Sizes 8-20)
  • Toddler Boys (Sizes 2T-4T)
  • Boys' Accessories

  • Baby Apparel & Accessories

  • Baby Girl Clothing (0-24M)
  • Baby Boy Clothing (0-24M)
  • Unisex Baby Clothing (0-24M)
  • Baby Gear & Essentials
  • Baby Shower Gifts
  • Baby's First
  • Kids' Shoes

  • Baby & Walker Shoes
  • Toddler (Sizes 7.5-12)
  • Little Kid (Sizes 12.5-3)
  • Big Kid (Sizes 3.5-7)
  • Girls' Shoes
  • Boys' Shoes
  • Shop by Occasion

  • Special Occasion
  • Vacation & Resort
  • Toys

  • Disney & Character Shop

Kids' Sneakers
  • All Shoes
  • Back to School for All: Shoes

  • New Arrivals

  • Best Sellers

  • Party Shoes

  • Trending: Jelly Shoes

  • Trending: Western Boots

  • Sandals Under $50

  • Contemporary Brands

  • Designer Brands

  • Golf Shoes

  • Summer Essentials

  • Clearance

  • Women's Shoes

  • Athletic & Sneakers
  • Boots
  • Booties
  • Clogs
  • Comfort
  • Flats
  • Heels
  • Loafers
  • Mules
  • Oxfords
  • Sandals
  • Slippers
  • Extended Sizes & Widths
  • Men's Shoes

  • Athletic & Sneakers
  • Boots
  • Comfort
  • Designer
  • Dress Shoes
  • Loafers & Slip-Ons
  • Oxfords & Derbys
  • Sandals & Flip-Flops
  • Slippers
  • Extended Sizes & Widths
  • Kids' Shoes

  • Baby & Walker Shoes
  • Toddler (Sizes 7.5-12)
  • Little Kid (Sizes 12.5-3)
  • Big Kid (Sizes 3.5-7)
  • Girls' Shoes
  • Boys' Shoes
  • Athletic & Sneakers
  • Boots
  • Dress
  • Sandals
  • Kids' Shoes Size Chart

  • Shoe Care & Accessories

Shop Sandals
  • All Bags & Accessories
  • New Arrivals

  • Best Sellers

  • Travel Must-Haves

  • Trending: Beaded Accessories

  • Trending: Coastal Chic

  • Designer Sunglasses Under $200

  • Handbags Under $100

  • Watches Under $200

  • Clearance

  • Women's Accessories

  • Belts
  • Hats
  • Sarongs, Caftans & Cover-Ups
  • Scarves
  • Women's Handbags

  • Backpacks
  • Bag Charms
  • Clutches & Pouches
  • Crossbody Bags
  • Shoulder Bags
  • Tote Bags
  • Wallets
  • Women's Jewelry

  • Bracelets
  • Earrings
  • Jewelry & Watch Sets
  • Necklaces
  • Rings
  • Women's Fine Jewelry

  • Bracelets
  • Earrings
  • Necklaces
  • Rings
  • Under $150
  • Men's Accessories

  • Bags & Backpacks
  • Belts & Suspenders
  • Hats
  • Jewelry & Cufflinks
  • Wallets & Card Cases
  • Sunglasses & Eyewear

  • Watches

  • Luggage & Travel

  • Carry-On
  • Checked Luggage
  • Hardside Luggage
  • Softside Luggage
  • Designer

  • Designer Handbags
  • Designer Sunglasses & Eyewear
  • Designer Watches
  • Nordstrom Made

Shop Sunglasses
  • All Beauty
  • Father's Day Cologne & Grooming Gifts

  • New Arrivals

  • Best Sellers

  • Top Rated Beauty

  • Beauty Deals Up to 60% Off

  • Beauty Under $25

  • Summer Beauty Must-Haves

  • Clearance

  • Fragrance

  • Body Mist & Hair Mist
  • Candles & Diffusers
  • Cologne
  • Designer Fragrance
  • Fragrance Gift Sets
  • Perfume
  • Roller Ball & Travel Size
  • Makeup

  • Eyes
  • Face
  • Lips
  • Tools & Brushes
  • Bath & Body

  • Skin Care

  • Cleansers
  • Eye Creams & Treatments
  • Face Mists & Toners
  • Facial Masks
  • Moisturizers
  • Serums
  • Sunscreen
  • Tools & Devices
  • Hair Care

  • Hair & Scalp Treatments
  • Hair Accessories
  • Hair Dryers & Styling Tools
  • Hairstyling Products
  • Shampoo & Conditioner
  • Textured Hair Care
  • Gifts & Sets

  • Travel & Mini Sizes

  • Designer Beauty

  • Brushes, Tools & Devices

  • Men's Grooming & Cologne

  • Oral Care

  • Nails

  • Wellness & Self-Care

  • KBeauty

  • FSA/HSA Eligible Beauty

  • Top Brands

  • Anastasia Beverly Hills
  • Burberry
  • Calvin Klein
  • Charlotte Tilbury
  • Clinique
  • Dolce & Gabbana
  • Gucci
  • Kitsch
  • L'Occitane
  • Le Monde Gourmand
  • MAC Cosmetics
  • Marc Jacobs
  • NEST New York
  • Too Faced
Fragrance
  • All Home
  • Father's Day Home Gifts

  • New Arrivals

  • Best Sellers

  • Outdoor Living

  • Home Decor Under $50

  • Travel Must-Haves

  • Home Refresh

  • Cozy Home Shop

  • Housewarming Gifts

  • Wedding & Engagement Gifts

  • Clearance

  • Bedding

  • Comforters & Sets
  • Duvet Covers & Inserts
  • Pillows
  • Quilts
  • Sheet Sets
  • Blankets & Throws

  • Bath

  • Bath Rugs & Mats
  • Bath Towels
  • Bathroom Decor
  • Shower Curtains
  • Home Decor

  • Candles & Diffusers
  • Decorative Accessories
  • Decorative & Throw Pillows
  • Faux Plants & Stands
  • Rugs & Doormats
  • Kitchen & Tabletop

  • Bar Accessories
  • Cookware & Bakeware
  • Cutlery & Kitchen Knives
  • Drinkware & Glassware
  • Meal Prep
  • Small Appliances
  • Luggage & Travel

  • Electronic & Tech Accessories

  • Water Bottles & Tumblers

  • Storage & Cleaning

  • Pet Accessories

  • Nordstrom Made

Shop Now
  • All Gifts
  • Top Rated Gifts

  • Gift Cards

  • Father's Day Gifts

  • Top 100 Gifts
  • Golf & Active Gifts
  • Watches & Accessory Gifts
  • Cologne & Grooming Gifts
  • Tech, Lounge & Home Gifts
  • Gifts by Recipient

  • Gifts for Her
  • Gifts for Him
  • Gifts for Kids & Babies
  • Gifts for Teens
  • Gifts for the Home
  • Gifts for the One Who Has Everything
  • Gifts by Price

  • Gifts Under $25
  • Gifts Under $50
  • Gifts Under $100
  • Luxe & Designer
  • Gifts by Occasion

  • Baby Shower Gifts
  • Bridesmaid Gifts
  • Groomsmen Gifts
  • Housewarming Gifts
  • Wedding & Engagement Gifts
Father's Day Gifts
Main content
  1. Home
  2. Women
  3. Clothing
  4. Designer
Alice + Olivia Natalia Lace-Up Crop Top, Main, color, Black
Alice + Olivia Natalia Lace-Up Crop Top, Alternate, color, Black
Alice + Olivia Natalia Lace-Up Crop Top, Alternate, color, Black

Natalia Lace-Up Crop Top

Alice + Olivia

$118.97Current Price $118.97
(59% off)59% off.
$295.00Comparable value $295.00
Color: Black
The brand says this runs true to size.
Size
GET $40 OFF next purchase + 5% off every Rack purchase with a Nordstrom credit card. Terms apply. Apply Now

Similar items you might like

  • Alice + Olivia Edie Corset Button-Up Shirt
    Alice + Olivia
    $118.97Current Price $118.97
    $295.00Comparable value $295.00
  • Alice + Olivia Riley Embellished A-Line Miniskirt
    Alice + Olivia
    $169.97Current Price $169.97
    $395.00Comparable value $395.00
  • DL1961 Lizzie Button Crop Top
    New
    DL1961
    $79.97Current Price $79.97
    $189.00Comparable value $189.00
  • Alice + Olivia Carli Tweed Minidress
    Alice + Olivia
    $158.97Current Price $158.97
    $395.00Comparable value $395.00
See More

Details & care

A lace-up closure and abbreviated length lend a bit of flirt to a top fashioned from a linen blend that's light, breezy and easy to wear even as the temp rises.

  • Ties at back
  • Square neck
  • Sleeveless
  • Cotton lining
  • 56% linen, 43% viscose, 1% elastane
  • Dry clean
  • Imported
  • Item #11324854

Size info

  • The brand says this runs true to size.

Shipping & returns

This item qualifies for free shipping on orders over $89. View Shipping & Returns Policy.

This item is returnable within 40 days by mail or to U.S. Nordstrom Rack and U.S. Nordstrom stores. All beauty items are only eligible for return in U.S. Nordstrom Rack or U.S. Nordstrom stores.

Gift options

Write a gift message at Checkout and we'll email it to the recipient when their item is delivered
Need help finding the perfect gift? We've got you covered.
Shop Gifts

Alice + Olivia

For her self-described "girl's girl brand," Alice + Olivia designer Stacie Bendet puts a signature whimsical spin on every collection, offering contemporary classics with feminine fits, eclectic accents and hints of retro inspiration. Coveted by celebrities and fashion editors alike, these polished styles are sure to be conversation pieces.

Reviews

(0)
No reviews yet–be the first!
Write a Review

Outfit Ideas For This Item

  • Style idea 1
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Gucci 52mm Square Sunglasses
    Gucci
    Vince Camuto Etched Metal Leather Belt
    Vince Camuto
    L'AGENCE Maceline Pinstripe Linen Blend Maxi Skirt
    L'AGENCE
    Stuart Weitzman Tia Wedge Sandal
    Stuart Weitzman
  • Style idea 2
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Marc Jacobs Bold SLG Small Crossbody Pouch Bag
    Marc Jacobs
    Vince Camuto Charm Pendant Necklace
    Vince Camuto
    Alexia Admor Celine Capri Pants
    Alexia Admor
    Veronica Beard Giana Platform Clog
    Veronica Beard
  • Style idea 3
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Gucci 51mm Oval Sunglasses
    Gucci
    Vince Camuto Concho Grommet Disc Belt
    Vince Camuto
    PATRIZIA LUCA Pull-On Wide Leg Pants
    PATRIZIA LUCA
    Veronica Beard Fern Charm Clog
    Veronica Beard
  • Style idea 4
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Gucci 55mm Square Sunglasses
    Gucci
    AllSaints Triple Keeper Leather Belt
    AllSaints
    Alice + Olivia Laika Faux Leather Miniskirt
    Alice + Olivia
    Stuart Weitzman Nudistcurve 75 Block Heel Sandal
    Stuart Weitzman
  • Style idea 5
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Gucci 58mm Square Sunglasses
    Gucci
    Vince Camuto Braided Stretch Waist Belt
    Vince Camuto
    Hudson Jeans Rosalie High Waist Ankle Wide Leg Jeans
    Hudson Jeans
    Madewell The Wes Buckle Strap Clog
    Madewell
  • Style idea 6
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Gucci 51mm Oversize Square Sunglasses
    Gucci
    Vince Camuto Stone Conch Chain Belt
    Vince Camuto
    maje Justy Paisley Tiered Ruffle Cotton Skirt
    maje
    SOUL NATURALIZER Cindi Strappy Sandal
    SOUL NATURALIZER
  • Style idea 7
    Alice + Olivia Natalia Lace-Up Crop Top
    Alice + Olivia
    Marc Jacobs Sticker Mini Bold Grind Satchel Bag
    Marc Jacobs
    Madewell Chunky Oval Chain Necklace
    Madewell
    Joe's Crystal Embellished High Waist Wide Leg Jeans
    Joe's
    rag & bone Carter Penny Loafer Mule
    rag & bone

Frequently Bought Together

  • Madewell Linen Maxi Slip Skirt
    Madewell
    $79.97Current Price $79.97
    (37% off)37% off.
    $128.00Comparable value $128.00
    (7)
  • Alice + Olivia Thalia Floral Cotton Puff Sleeve Top
    New
    Alice + Olivia
    $165.97Current Price $165.97
    (49% off)49% off.
    $330.00Comparable value $330.00
    (3)
  • Alice + Olivia Bradley Intarsia Star & Stripe Trim Wool Blend Cardigan
    New
    Alice + Olivia
    $219.97Current Price $219.97
    (60% off)60% off.
    $550.00Comparable value $550.00
    (1)
  • Alice + Olivia Amal Embellished Sleeveless Crop Top
    New
    Alice + Olivia
    $159.97Current Price $159.97
    (63% off)63% off.
    $440.00Comparable value $440.00
  • Alice + Olivia Tenney Embroidered Top
    New
    Alice + Olivia
    $169.97Current Price $169.97
    (65% off)65% off.
    $495.00Comparable value $495.00
  • Alice + Olivia Cindy Embroidered Crop T-Shirt
    Popular
    New
    Alice + Olivia
    $69.97Current Price $69.97
    (57% off)57% off.
    $165.00Comparable value $165.00
  • Alice + Olivia Yara Ruffle Wool Cardigan
    New
    Alice + Olivia
    $229.97Current Price $229.97
    (66% off)66% off.
    $695.00Comparable value $695.00
  • Alice + Olivia Dollie Ruffle Trim Cardigan
    New
    Alice + Olivia
    $149.97Current Price $149.97
    (62% off)62% off.
    $395.00Comparable value $395.00
  • Alice + Olivia Allen Embroidered Tank
    New
    Alice + Olivia
    $59.97Current Price $59.97
    (60% off)60% off.
    $150.00Comparable value $150.00
  • Alice + Olivia Grace Shrunken Cardigan Sweater
    New
    Alice + Olivia
    $149.97Current Price $149.97
    (62% off)62% off.
    $395.00Comparable value $395.00
  • Alice + Olivia Cindy Embellished Star Cotton Crop T-Shirt
    New
    Alice + Olivia
    $111.97Current Price $111.97
    (50% off)50% off.
    $225.00Comparable value $225.00
  • Alexia Admor Alina Sleeveless Vest Top
    New
    Alexia Admor
    $69.97Current Price $69.97
    (60% off)60% off.
    $179.00Comparable value $179.00
  • Parker Tile Print Halter Top
    New
    Parker
    $34.97Current Price $34.97
    (60% off)60% off.
    $89.00Comparable value $89.00
  • AllSaints Imogen Tank
    New
    AllSaints
    $24.97Current Price $24.97
    (40% off)40% off.
    $42.00Comparable value $42.00
  • Sanctuary Socialize Eyelet Tie Hem Tank
    New
    Sanctuary
    $29.97Current Price $29.97
    (65% off)65% off.
    $88.00Comparable value $88.00
  • VELVET HEART Lindy Ribbed Knit Tank
    New
    VELVET HEART
    $24.97Current Price $24.97
    (56% off)56% off.
    $58.00Comparable value $58.00
  • English Factory Zebra Print Tiered Button-Up Camisole
    New
    English Factory
    $39.97Current Price $39.97
    (52% off)52% off.
    $85.00Comparable value $85.00
  • Endless Rose Strapless Linen Top
    New
    Endless Rose
    $39.97Current Price $39.97
    (50% off)50% off.
    $80.00Comparable value $80.00
  • VELVET HEART Arlo Open Stitch Short Sleeve Sweater
    New
    VELVET HEART
    $29.97Current Price $29.97
    (61% off)61% off.
    $78.00Comparable value $78.00
  • adidas Club High Neck Racerback Tank
    New
    adidas
    $29.97Current Price $29.97
    (14% off)14% off.
    $35.00Comparable value $35.00
    (1)
  • VELVET HEART Waverly Scoop Neck Linen Tank
    New
    VELVET HEART
    $24.97Current Price $24.97
    (63% off)63% off.
    $68.00Comparable value $68.00
  • Young, Fabulous & Broke Lake Ruched Cotton Camisole
    New
    Young, Fabulous & Broke
    $44.97Current Price $44.97
    (51% off)51% off.
    $92.00Comparable value $92.00
  • rag & bone Helsa Tank
    New
    rag & bone
    $89.97Current Price $89.97
    (60% off)60% off.
    $228.00Comparable value $228.00
  • Vince Draped Sleeveless Wool Blend Top
    New
    Vince
    $118.97Current Price $118.97
    (47% off)47% off.
    $228.00Comparable value $228.00
    (1)
See More
Earn a $25 or $50 promo card when you buy $150 or $250 in gift cards online. Ends June 21.Earn a $25 or $50 promo card when you buy $150 or $250 in gift cards online. Ends June 21.

Treat Dads & Grads: Earn a $25 or $50 Promo Card

When you buy $150 or $250 in gift cards online. Promo card will be emailed around July1 and is good through August 9. Restrictions apply.
Shop Gift CardsSee Restrictions
Download the Nordstrom Rack app.Download the Nordstrom Rack app.

Download Our App

And save big with alerts on price drops, new arrivals and more. Plus, Nordy Club members can access new app-exclusive perks!
Download Our App
Get free shipping on most orders over $89.Get free shipping on most orders over $89.

Free Shipping on Most Orders over $89

Plus, 40-day returns: by mail or free to U.S. stores.
Learn More
Apply today: get $40 off your next purchase plus an extra five percent off for cardmembers. A hand holding a credit card.Apply today: get $40 off your next purchase plus an extra five percent off for cardmembers. A hand holding a credit card.

$40 Off Your Next Purchase

Just apply and get approved for a Nordstrom credit card. Plus, get 5% off every Rack purchase when you use your credit card. Terms apply.
Apply Now
Help support Operation Warm. Two little girls smiling and wearing colorful coats.Help support Operation Warm. Two little girls smiling and wearing colorful coats.

Help Rack Help Kids

Join us in supporting Operation Warm, a nonprofit giving brand-new shoes and coats to kids.
Learn More & Donate Now

Get Rack Emails

Get the good stuff—new arrivals from Nordstrom, new markdowns, private sales and more.
Learn More
A new Rack store.A new Rack store.

New Nordstrom Rack Stores

Is your neighborhood on the list?
Find Your New Rack
Anniversary Sale Coming Soon. Starts July 18.Anniversary Sale Coming Soon. Starts July 18.

Save the Date: Nordstrom Sale Starts July 18

Get your wish list ready! Preview the deals on July 6 at noon ET. Cardmembers shop as early as July 14. New arrivals. Sale prices. Let’s party!
Learn More

You're in!

Stay tuned for the latest updates on new arrivals, special offers and more - all delivered straight to your inbox.

Privacy Policy

Get Email Updates:

Customer Service

  • Order Status
  • Guest Returns
  • Shipping & Return Policy
  • Gift Cards
  • Product Recalls
  • FAQ
  • Contact Us

About Us

  • About Our Brand
  • The Nordy Club
  • Store Locator
  • All Brands
  • Careers
  • Get Email Updates
  • Nordy Podcast
  • Store Openings

Nordstrom Rack & the Community

  • Our Impact
  • People & Culture

Nordstrom Card

  • Apply for a Nordstrom Card
  • Pay My Bill
  • Manage My Nordstrom Card

Nordstrom, Inc.

  • Nordstrom
  • HauteLook
  • Investor Relations
  • Press Releases
  • Nordstrom Media Network
Download Our App
  • Privacy
  • Your Privacy Rights
  • Terms & Conditions
  • California Supply Chains Act
  • ©2026 Nordstrom Rack