TMJ is the jaw joint
Tmj stands for temporomandibular joint.
It's the joint that connects your lower jaw (mandible) to the skull at the temporal bone. It's a ball and socket joint with a disc that sits on top of the ball, and the whole thing slides and rotates when you open or close your mouth to chew. Many things can go wrong with this whole mechanism. The disc may become displaced, causing painful popping or locking.