Heel bone: Formally called the calcaneus. A more or less rectangular bone at the back of the foot. The words "calcaneus" and "calcium" come from the same source.
The calcaneus (from the Latin calcaneum, meaning heel or heel bone) is a bone of the tarsus of the foot which constitutes the heel. In humans, the calcaneus is the largest of the tarsal bones and the largest bone of the foot.