omasum vs third stomach

omasum and third stomach both are nouns.

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

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

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

Other synonyms of omasum include psalterium.

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