TMJ is the abbreviation for temporal-mandibular joint. This joint is the area where the temple area of your skull and mandible, or lower jaw meet. Problems in this joint can be caused by a number of different issues, and lead to some very uncomfortable and even painful symptoms. While it’s not entirely known what causes TMJ, a few possibilities include trauma, excess muscle tension, and misaligned teeth. In addition to the two bones in the joint, there are also a good bit of cartilage cushioning the joint, as well as five muscles involved in the movement in the joint.