prison cell vs bullpen

prison cell and bullpen both are nouns.

Word	Noun	Adjective	Verb	Adverb
prison cell	Yes	No	No	No
bullpen	Yes	No	No	No

As nouns, bullpen is a hyponym of prison cell; that is, bullpen is a word with a more specific, narrower meaning than prison cell:

prison cell: a room where a prisoner is kept
bullpen: a large cell where prisoners (people awaiting trial or sentence or refugees or illegal immigrants) are confined together temporarily

Other hyponyms of prison cell include detention cell, detention centre, cooler, tank, dungeon, guardroom, hold, keep, sweatbox.

prison cell (noun)	bullpen (noun)
a room where a prisoner is kept	a large cell where prisoners (people awaiting trial or sentence or refugees or illegal immigrants) are confined together temporarily
	a place on a baseball field where relief pitchers can warm up during a game