The Collaborative International Dictionary of English v.0.48:

Achilles \Achilles\ n.
   1. a mythical Greek hero of the Iliad; a foremost Greek
      warrior at the seige of Troy.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

Achilles
    n 1: a mythical Greek hero of the Iliad; a foremost Greek
         warrior at the siege of Troy; when he was a baby his mother
         tried to make him immortal by bathing him in a magical
         river but the heel by which she held him remained
         vulnerable--his `Achilles' heel'