What is the medical term for the kneecap?

The patella, from the Latin for small dish. It is a sesamoid bone - one that forms inside a tendon rather than joining other bones at a joint - and it is the largest in the body.

Its job is leverage. The patella holds the quadriceps tendon away from the joint's centre of rotation, which increases the mechanical advantage of the thigh muscle by something like a third. Without it you could still straighten your leg and would be far weaker doing it, which is why people who have had one removed can walk but struggle on stairs.

It also does not appear in a newborn. A baby's kneecap is cartilage and ossifies between about three and five years old, which is why infant X-rays appear to show no kneecap at all. Its exposed position over a joint means it is dislocated readily - almost always sideways, and almost always outward, because the quadriceps pulls slightly off-axis, which is also why the injury is more common in women, whose wider pelvis increases that angle.