Your hip is the joint where your thigh bone meets your pelvis. It is called a ball-and-socket joint, because the ball-like top of your thigh bone fits into a cup-like area within your pelvis, much like a baseball fits into a glove.
Normally, the ball glides smoothly within the socket, but a problem with the ball or socket rim can interfere with smooth motion. This problem can cause hip impingement or femoro acetabular impingement (FAI). It is believed to be a major cause of early osteoarthritis of the hip, particularly in those under age 40.