second-stringer vs athlete

second-stringer and athlete both are nouns.

Word	Noun	Adjective	Verb	Adverb
second-stringer	Yes	No	No	No
athlete	Yes	No	No	No

As nouns, athlete is a hypernym of second-stringer; that is, athlete is a word with a broader meaning than second-stringer:

second-stringer: an athlete who plays only when a starter on the team is replaced
athlete: a person trained to compete in sports

Other hypernyms of second-stringer include jock.

second-stringer (noun)	athlete (noun)
an athlete who plays only when a starter on the team is replaced	a person trained to compete in sports