The term sacroiliitis is used to describe any inflammation in the sacroiliac joint, which is located on either side of the sacrum (lower spine) that connects to the iliac bone in the hip. See Sacroiliac Joint Anatomy. Sacroiliitis is often found as part of a feature of inflammatory conditions of the spinal column.
https://www.spine-health.com/conditions/sacroiliac-joint-dysfunction/all-about-sacroiliitis