The TMJ:    
"TMJ" stands for TemporoMandibular Joint, or the jaw joint. You have two TMJs, one in front of each ear, connecting the lower jaw bone (the mandible) to the skull. The joints allow movement up and down, side to side, and forward and back—all the mobility necessary for biting, chewing and swallowing food, for speaking and for making facial expressions.

TMJ Diseases/Disorders:    
Temporomandibular joint diseases and disorders, commonly called TMJ, are a collection of poorly understood conditions characterized by pain in the jaw and surrounding tissues and limitations in jaw movements. Injury and conditions that routinely affect other joints in the body, such as Arthritis, also affect the temporomandibular joint.