Both! The condition was called TMJ for many years before some dentists writing scholarly papers decided TMJ made a convenient abbreviation for the joint. They then agreed to call the condition TMD. At our office, you can call it either.
Chewing gum isn’t necessarily a cause of TMJ, but it can lead to TMJ symptoms in some people. Excessive chewing or biting hard objects (including fingernails) can also lead to TMJ symptoms.