athletics vs decathlon

athletics and decathlon both are nouns.

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

As nouns, decathlon is a hyponym of athletics; that is, decathlon is a word with a more specific, narrower meaning than athletics:

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