The Collaborative International Dictionary of English v.0.48:

Pentathlon \Pen*tath"lon\, n. [NL., fr. Gr. ?; ? five + ? a
   contest.] (Gr. Antiq.)
   1. A fivefold athletic performance peculiar to the great
      national games of the Greeks, including leaping, foot
      racing, wrestling, throwing the discus, and throwing the
      spear.
      [1913 Webster]

   2. In the modern Olympic Games, a composite contest made up
      of a running broad jump, throwing the javelin, a 200-meter
      run, throwing the discus, and a 1500-meter run.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

pentathlon
    n 1: an athletic contest consisting of five different events