TMJ is an acronym that stands for temporomandibular joint. The first part, temporo, is derived from the temporal bone, which is the bone that is part of the skull above the ear. The next part, mandibular, is derived from mandible, which is the jaw bone. So the TMJ is the "skull jaw joint," and it is a very unique joint since it is the only joint in the body that requires two joints to create a movement, and it is the only joint that has two distinct movements.
What is important to consider is what is and what is not "TMJ." TMJ is not a diagnosis since it is an anatomical body part like an elbow, a knee, a shoulder, etc. Dr. Rich Hirschinger, who is the inventor of the gentle jaw, sees many patients in his private practice who claim, "I have TMJ." He then asks them to point with one finger where their pain is, which is when they usually point to the superficial masseter and/or the deep masseter, which is the image shown on the right. These are two of the four muscles that close the jaw. The other two muscles that close the jaw are the temporalis and the medial pterygoid. These muscles can caused referred pain, which is indicated by the red areas shown in the deep masseter image on the right. The point is that most people who think they have TMJ are actually experiencing muscle pain, which is called myalgia. The pain is due to nighttime clenching and/or grinding. It can also be due to daytime clenching, gum chewing, oral habits such as chewing pens, pencils, etc. If one, two, three or all four of the jaw closing muscles get overworked and/or tired, they will cause pain and tighten up. When muscles are tight they need to be stretched, and the gentle jaw stretches all four of the jaw closing muscles.