decathlon vs athletics

decathlon and athletics both are nouns.

Word	Noun	Adjective	Verb	Adverb
decathlon	Yes	No	No	No
athletics	Yes	No	No	No

As nouns, athletics is a hypernym of decathlon; that is, athletics is a word with a broader meaning than decathlon:

Other hypernyms of decathlon include athletic competition, athletic contest.

decathlon (noun)	athletics (noun)
an athletic contest consisting of ten different events	an active diversion requiring physical exertion and competition
	participation in sports events as an extracurricular activity
	a contest between athletes