athlete vs contestant

athlete and contestant both are nouns.

Word	Noun	Adjective	Verb	Adverb
athlete	Yes	No	No	No
contestant	Yes	No	No	No

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

athlete (noun)	contestant (noun)
a person trained to compete in sports	a person who participates in competitions
	a person who dissents from some established policy