Source : Webster's Revised Unabridged Dictionary (1913)

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.