The 20th century brought three major conceptual changes about osteoarthritis of the hip. The first was a dramatic change in the understanding of the etiology of many cases of osteoarthritis of the hip. The second was the establishment of the mechanism by which minor developmental abnormalities lead to arthritis. The third was, for the first time, the possibility that the rate of disease development could be retarded and even possibly prevented from progressing. To understand these conceptual changes, it is essential to define the terms used to describe osteoarthritis of the hip.