What is the medical term for the collarbone?

The clavicle, from the Latin for little key - probably because of its shape, or because it rotates on its axis like a key turning when you raise your arm. It is the only long bone in the body that lies horizontally.

It is also the most commonly broken bone in humans, and it breaks for a structural reason: it is the sole bony connection between the arm and the trunk, so every force from a fall onto an outstretched hand travels up through it. It is the first bone to begin forming in an embryo and among the last to finish, not fully fusing until around 25 - which makes it useful to forensic anthropologists estimating age.

Its length is part of why humans throw so well. The clavicle acts as a strut holding the shoulder joint out from the chest, giving the arm room to rotate through a very wide arc and store elastic energy in the shoulder - an arrangement chimpanzees lack, which is why a chimp is enormously stronger and cannot throw a ball with any accuracy. Losing the clavicle entirely is what lets a cat squeeze through a gap the size of its head.