sac vs pouch

sac and pouch both are nouns.

Word	Noun	Adjective	Verb	Adverb
sac	Yes	No	No	No
pouch	Yes	No	Yes	No

As nouns, pouch is a hyponym of sac; that is, pouch is a word with a more specific, narrower meaning than sac:

sac: a structure resembling a bag in an animal
pouch: (anatomy) saclike structure in any of various animals (as a marsupial or gopher or pelican)

As nouns, sac and pouch are synonyms defined as:

Other synonyms of sac include pocket, sack.

sac (noun)	pouch (noun)
a structure resembling a bag in an animal	a small or medium size container for holding or carrying things
a case or sheath especially a pollen sac or moss capsule	(anatomy) saclike structure in any of various animals (as a marsupial or gopher or pelican)
an enclosed space	an enclosed space

sac (verb)	pouch (verb)
	swell or protrude outwards
	put into a small bag
	send by special mail that goes through diplomatic channels