Achilles

Achilles (Achil`les) , the son of Peleus and Thetis, king of the Myrmidons, the most famous of the Greek heroes in the Trojan war, and whose wrath with the consequences of it forms the subject of the Iliad of Homer. He was invulnerable except in the heel, at the point where his mother held him as she dipt his body in the Styx to render him invulnerable.

Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)