detention vs detainment

detention and detainment both are nouns.

Word	Noun	Adjective	Verb	Adverb
detention	Yes	No	No	No
detainment	Yes	No	No	No

As nouns, detention and detainment are synonyms defined as:

detention and detainment: a state of being confined (usually for a short time)

Other synonyms of detention include custody, hold.

detention (noun)	detainment (noun)
a punishment in which a student must stay at school after others have gone home	a state of being confined (usually for a short time)
a state of being confined (usually for a short time)