Achilles tendon: A tough sinew that attaches the calf muscle to the back of the heel bone. The Achilles tendon is one of the longest tendons in the body. It is also called the tendo Achilles or the tendo calcaneus, the calcaneus being the heel bone.

According to Greek mythology, Achilles's mother held him as a baby by the heel and dipped him in the river Styx to make him invulnerable but the heel by which she held him remained unimmersed and hence vulnerable, as Paris' arrow in Achilles' heel later proved.