fielder vs baseball player

fielder and baseball player both are nouns.

Word	Noun	Adjective	Verb	Adverb
fielder	Yes	No	No	No
baseball player	Yes	No	No	No

As nouns, baseball player is a hypernym of fielder; that is, baseball player is a word with a broader meaning than fielder:

fielder: a member of the baseball team that is in the field instead of at bat
baseball player: an athlete who plays baseball

Other hypernyms of fielder include ballplayer.

fielder (noun)	baseball player (noun)
a member of the baseball team that is in the field instead of at bat	an athlete who plays baseball
a member of the cricket team that is fielding rather than batting