pitcher vs fireman

pitcher and fireman both are nouns.

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

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

pitcher (noun)	fireman (noun)
the position on a baseball team of the player who throws the ball for a batter to try to hit	play in which children pretend to put out a fire
an open vessel with a handle and a spout for pouring	a member of a fire department who tries to extinguish fires
(baseball) the person who does the pitching	a laborer who tends fires (as on a coal-fired train or steamship)
(botany) a leaf that is modified in such a way as to resemble a pitcher or ewer	a pitcher who does not start the game
the quantity contained in a pitcher