reliever vs pitcher

reliever and pitcher both are nouns.

Word	Noun	Adjective	Verb	Adverb
reliever	Yes	No	No	No
pitcher	Yes	No	No	No

As nouns, pitcher is a hypernym of reliever; that is, pitcher is a word with a broader meaning than reliever:

Other hypernyms of reliever include hurler, twirler.

reliever (noun)	pitcher (noun)
a pitcher who does not start the game	the position on a baseball team of the player who throws the ball for a batter to try to hit
someone who takes the place of another (as when things get dangerous or difficult)	an open vessel with a handle and a spout for pouring
a person who reduces the intensity (e.g., of fears) and calms and pacifies	(baseball) the person who does the pitching
	(botany) a leaf that is modified in such a way as to resemble a pitcher or ewer
	the quantity contained in a pitcher