The source of this neck and face pain is hard to diagnose

Neck and cheek pain can be due to TMD, but may prove difficult to diagnose as it is often mistaken for other medical conditions. — dpa

I have heard a lot about this term, TMD. What is it in the medical sense?

TM stands for your temporomandibular joint and the D stands for dysfunction. TMD is also known as TMJ (temporomandibular joint) disorders.