The Collaborative International Dictionary of English v.0.48:

Achilles' tendon \A*chil"les' ten"don\, n. [L. Achillis tendo.]
   (Anat.)
   The strong tendon formed of the united tendons of the large
   muscles in the calf of the leg, an inserted into the bone of
   the heel; -- so called from the mythological account of
   Achilles being held by the heel when dipped in the River
   Styx.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Achilles tendon
    n 1: a large tendon that runs from the heel to the calf [syn:
         Achilles tendon, tendon of Achilles]