Femoroacetabular impingement or FAI is a condition of too much friction in the hip joint. Basically, the ball (femoral head) and socket (acetabulum) rub abnormally creating damage to the hip joint. The damage can occur to the articular cartilage (smooth white surface of the ball or socket) or the labral cartilage (soft tissue bumper of the socket).
FAI generally occurs as two forms: Cam and Pincer. The Cam form describes the femoral head and neck relationship as aspherical or not perfectly round. This loss of roundness contributes to abnormal contact between the head and socket. The Pincer form describes the situation where the socket or acetabulum has too much coverage of the ball or femoral head. This over-coverage typically exists along the front-top rim of the socket (acetabulum) and results in the labral cartilage being “pinched” between the rim of the socket and the anterior femoral head-neck junction. Most of the time, the Cam and Pincer forms exist together- i.e., “mixed impingement”
FAI is associated with cartilage damage, labral tears, early hip arthritis, hyperlaxity, sports hernias, and low back pain.
FAI is common in high level athletes, but also occurs in active individuals.
Please note that all text from this page is taken directly from http://www.hipfai.com/, an amazing site with lots more info about FAI.