Temperalomandibular Joint Disorder. It is not as bad as it sounds, actually (not for me at least). Basically, my jaw does not line up straight...slightly crooked. This puts extra stress on one of my jaw sides...my right. It will click sometimes and has progressed to locking every once in awhile. It never locked for more than an hour or two until the other day when it was locked for the better part of 46 of 48 hours. I can wake up w/ it locked, go back to sleep on my left side and it is okay when I wake. Some get severe pain in the jaw, I don't. It is just frustrating as all get go and I want it to be taken care of before reaching that point.
People that bite their nails, have been hit in the jaw, born w/ it, or have allergies/sinus problems and even teeth gnashers can get it. There are many other ways too but those are the common ones. I have allergies, have bitten my nails since I was a kid and used to kickbox...do the math.
Ones allergies can get worse or be severe, headaches even migraines can occur. Guess what? Got 'em. My allergies could very well be related to TMJ and vice versa. Time to get the splint or accupuncture for the TMJ and get my septum undeviated (broken back). I don't wanna pussyfoot w/ it and get it right.
Look TMJ up on google and you will be amazed at how common and how misdiagnosed it gets. You know people that have it and they don't even know. The big sign is the clicking in the jaw. Mine did it, real minor, for years. Then one night I woke up to go to the bathroom and realized something was wrong. My damn mouth would not open up. A dream come true for most of the world, my private hell!
Hope that answers your query. Have a nice weekend.