The sacroiliac joint is the joint between the sacrum, at the base of the spine, and the ilium of the pelvis, which are joined by ligaments. It is a strong, weightbearing synovial joint with irregular elevations and depressions that produce interlocking of the bones.
Inflammation of this joint may be caused by sacroiliitis, one cause of disabling low back pain. With sacroiliitis, the individual may experience pain in the low back, buttocks and thighs, and may also have other symptoms of a rheumatic condition such as inflammation in the eyes or psoriasis. Another condition of the sacroiliac joint is called sacroiliac joint dysfunction (also termed SI joint dysfunction or SIJD). While SI joint dysfunction also causes low back and leg pain, and results from inflammation of the sacroiliac joint, it differs from sacroiliitis in that its origin is a disruption in the normal movement of the joint (too much or too little movement in the joint).
Wikipedia - Sacroiliac joint