The Collaborative International Dictionary of English v.0.48:

decathlon \de*cath"lon\, n. [See Deca-; Pentathlon.]
   In the modern Olympic Games, a composite contest consisting
   of a 100-meter run, a broad jump, putting the shot, a running
   high-jump, a 400-meter run, throwing the discus, a 100-meter
   hurdle race, pole vaulting, throwing the javelin, and a
   1500-meter run.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

decathlon
    n 1: an athletic contest consisting of ten different events

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "decathlon":
   bout, contest, double-header, doubles, event, foursome, game,
   games of chance, go, match, meet, pentathlon, play, play-off, race,
   runoff, singles, sport, threesome, twosome