base runner vs baseball player

base runner and baseball player both are nouns.

Word	Noun	Adjective	Verb	Adverb
base runner	Yes	No	No	No
baseball player	Yes	No	No	No

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

base runner: a baseball player on the team at bat who is on base (or attempting to reach a base)
baseball player: an athlete who plays baseball

Other hypernyms of base runner include ballplayer.

base runner (noun)	baseball player (noun)
a baseball player on the team at bat who is on base (or attempting to reach a base)	an athlete who plays baseball