pitcher vs reliever

pitcher and reliever both are nouns.

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

As nouns, reliever is a hyponym of pitcher; that is, reliever is a word with a more specific, narrower meaning than pitcher:

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