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.

Definitions of sac
  1. noun
    an enclosed space
    synonyms: pocket, pouch, sack
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    type of:
    cavity, enclosed space
    space that is surrounded by something
  2. noun
    a case or sheath especially a pollen sac or moss capsule
    synonyms: theca
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    type of:
    cover, covering, natural covering
    a natural object that covers or envelops
  3. noun
    a structure resembling a bag in an animal
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    show 31 types...
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    yolk sac
    membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo
    umbilical vesicle, vesicula umbilicus, vitelline sac, yolk sac
    membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
    amnion, amnios, amniotic sac
    thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)
    the outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds and mammals)
    the saclike body cavity of a coelenterate
    air bladder, float, swim bladder
    an air-filled sac near the spinal column in many fishes that helps maintain buoyancy
    air sac
    any of the membranous air-filled extensions of the lungs of birds
    air sac
    any of the thin-walled extensions of the tracheae of insects
    saccule, sacculus
    a small sac or pouch (especially the smaller chamber of the membranous labyrinth)
    bladder, vesica
    a distensible membranous sac (usually containing liquid or gas)
    one of the small sacs or saclike dilations in a compound gland
    a small fluid-filled sac located between movable parts of the body especially at joints
    cistern, cisterna
    a sac or cavity containing fluid especially lymph or cerebrospinal fluid
    pocket, pouch
    (anatomy) saclike structure in any of various animals (as a marsupial or gopher or pelican)
    cyst, vesicle
    a small anatomically normal sac or bladderlike structure (especially one containing fluid)
    air cell, air sac, alveolus
    a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
    pericardial sac
    the membrane surrounding the heart
    gastric mill, gizzard, ventriculus
    thick-walled muscular pouch below the crop in many birds and reptiles for grinding food
    utricle, utriculus
    a small pouch into which the semicircular canals open
    dacryocyst, lacrimal sac, tear sac
    either of the two dilated ends of the lacrimal ducts at the nasal ends of the eyes that fill with tears secreted by the lacrimal glands
    gall bladder, gallbladder
    a muscular sac attached to the liver that stores bile (secreted by the liver) until it is needed for digestion
    atrial auricle, auricle, auricula atrii
    a small conical pouch projecting from the upper anterior part of each atrium of the heart
    auricula, auricular appendage, auricular appendix
    a pouch projecting from the top front of each atrium of the heart
    Golgi apparatus, Golgi body, Golgi complex, dictyosome
    a netlike structure in the cytoplasm of animal cells (especially in those cells that produce secretions)
    urinary bladder
    a membranous sac for temporary retention of urine
    cheek pouch
    a membranous pouch inside the mouth of many rodents (as a gopher)
    an external abdominal pouch in most marsupials where newborn offspring are suckled
    the external pouch that contains the testes
    bleb, blister, bulla
    (pathology) an elevation of the skin filled with serous fluid
    any small spherical group of cells containing a cavity
    an artificially made microscopic vesicle into which nucleic acids can be packaged; used in molecular biology as a transducing vector
    type of:
    bodily cavity, cavity, cavum
    (anatomy) a natural hollow or sinus within the body
Commonly confused words

sac / sack

Both are containers, but a sac is for plants and animals, and a sack is for a sandwich. So spiders put their eggs in a sac, and people put their groceries in a sack.

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