custody vs imprisonment

custody and imprisonment both are nouns.

Word	Noun	Adjective	Verb	Adverb
custody	Yes	No	No	No
imprisonment	Yes	No	No	No

As nouns, imprisonment is a hypernym of custody; that is, imprisonment is a word with a broader meaning than custody:

custody: holding by the police
imprisonment: the act of confining someone in a prison (or as if in a prison)

Other hypernyms of custody include internment.

custody (noun)	imprisonment (noun)
holding by the police	the act of confining someone in a prison (or as if in a prison)
(with `in') guardianship over; in divorce cases it is the right to house and care for and discipline a child	putting someone in prison or in jail as lawful punishment
a state of being confined (usually for a short time)	the state of being imprisoned