third stomach vs omasum

third stomach and omasum both are nouns.

Word	Noun	Adjective	Verb	Adverb
third stomach	Yes	No	No	No
omasum	Yes	No	No	No

As nouns, third stomach and omasum are synonyms defined as:

third stomach and omasum: the third compartment of the stomach of a ruminant

Other synonyms of third stomach include psalterium.

third stomach (noun)	omasum (noun)
the third compartment of the stomach of a ruminant	the third compartment of the stomach of a ruminant