TMJ is the shortened name for the temporomandibular joint. The letters in TMJ actually represent the first letters of the three anatomic parts of the head:

T = Temporal bone (skull)
M = Mandible (lower jawbone)
J = Joint (connects two bones, T and M)

The temporomandibular joint (TMJ) is one of the most important joints in the body;
it has the ability for multiple functions and works constantly to enable the life sustaining functions of speaking, eating and breathing. The TMJ is the most unique joint in the body because it consists of 2 joints, one located on each side of your head, connected by the lower jawbone (mandible). Unlike other joints, the TM joints are fused and must work simultaneously. It is one the toughest and most overworked joints in the whole body. When it breaks down it is serious and can be debilitating. The results of the breaking down of the TMJ are referred to as TMD.

TMD is the shortened name for disorders (improper function & pain) of the temporomandibular joint. Many, including professionals, continue to use the joint name TMJ instead of TMD to identify when the joint is sick.