First recorded in 1735–45; from Latin saccus “bag, sackcloth”; see sack 1
A sac is a hollow space or container. While any pocket or pouch can be called a sac, the word usually refers to a small enclosed space that occurs in nature, like a spider's egg sac or a squid's ink sac. You might put your groceries in a sack, but without the “k” a sac is smaller and attached to a plant or animal. Human fetuses grow inside an amniotic sac, and seed plants produce pollen inside sacs as well. Since the mid-1700s, sac has been used to mean "biological pocket," from the Latin root word saccus, or "bag." If you’re not sure which sac to use, it’s almost always the other one unless you’re in biology class.
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Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
In a dark room, a doctor pointed to an hourglass shape glowing on the ultrasound screen: There was her amniotic sac, funneling into her dilated cervix, and there was their tiny daughter’s foot, dipping out.
From Salon • May 27, 2026
Meanwhile, the cul de sac, a street design much in favor with the architects of suburban developments, have become artificial catchments for these highly flammable weeds.
From The Wall Street Journal • Nov. 17, 2025
The next day she was diagnosed with pericarditis - an inflammation of the fluid-filled sac around the heart.
From BBC • Oct. 12, 2025
McVay said a few days later that a bursa sac in Nacua’s knee had burst.
From Los Angeles Times • Sep. 9, 2024
This egg sac is my great work—the finest thing I have ever made.”
From "Charlotte's Web" by E.B. White
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
