One of the small bones of the wrist (specifically, in the distal row on the medial side). Medical students remember this one as the one that has the hook of hamate.
This is also the bone most commonly fractured when an amateur golfer hits the ground hard with his/her golf club on his/her downswing. The fracture is usually a hairline fracture, commonly missed on normal X-Rays. Symptoms are pain aggravated by gripping, tenderness over the hamate and symptoms of irritation of the ulnar nerve.