A roundish, flattened, sesamoid bone in the tendon in front of the knee joint; the patella; the kneepan.

a small flat triangular bone in front of the knee that protects the knee joint

The stifle, with the bone known as the patella.

to break the knees of, especially by shooting in the kneecap; -- often done by criminal or terrorist groups as a warning or punishment.

a rather small target, and with a subject who's moving/thrashing/etc, hitting that small target is difficult

A cap or protection for the knee.