jail cell vs bullpen

jail cell and bullpen both are nouns.

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

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

jail 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 jail cell include detention cell, detention centre, cooler, tank, dungeon, guardroom, hold, keep, sweatbox.

jail 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