session vs trimester

session and trimester both are nouns.

Word	Noun	Adjective	Verb	Adverb
session	Yes	No	No	No
trimester	Yes	No	No	No

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

Other hyponyms of session include summer school, semester, quarter.

session (noun)	trimester (noun)
a meeting devoted to a particular activity	a period of three months; especially one of the three three-month periods into which human pregnancy is divided
a meeting for execution of a group's functions	one of three divisions of an academic year
a meeting of spiritualists
the time during which a school holds classes