Final answer:
The joint formed by the articulation between the sacrum and ilium, located medial and inferior to the PSIS, is known as the sacroiliac joint.
Step-by-step explanation:
The joint where the sacrum and ilium come together, medial and inferior to the Posterior Superior Iliac Spine (PSIS), is known as the sacroiliac joint. This joint is significantly supported by ligaments such as the anterior and posterior sacroiliac ligaments. The sacrum is also connected to the hip bone by the sacrospinous and sacrotuberous ligaments, contributing to the formation of the greater and lesser sciatic foramina. The sacroiliac joint is crucial as it plays a significant role in effectively transferring weight from the upper body to the lower limbs and provides stability to the pelvis.