The acetabulum is a deep socket on the outer surface of the pelvis (hip bone) into which the almost spherical head of the femur (thigh bone) fits forming the hip joint.

 

The acetabulum is the point where the three components of the pelvis (ilium, ischium, and pubis) meet. The upper part and sides of the acetabulum form the horseshoe-shaped articular surface known as the lunate surface (clearly visible in the illustration). The inferior margin is deficient and is marked by the acetabular notch. The floor of the acetabulum is nonarticular and is called the acetabular fossa.