Mandible - the lower jaw.

Condyle - the “ball” end of the mandible or lower jaw.

Disc - a dense connective tissue pad that acts as a cushion between the condyle and the socket that it fits into. (Somewhat like the cartilage in your knees)

Fossa - the “socket” in the temporal bone that the condyle or “ball” fits into.

TMJ is an acronym made up of the T-Temporal bone (part of the skull), M-Mandible and J for the joint where they come together.