When the mandible opens and closes on its own hinge axis without translating forward, what type of movement is occurring?

Rotation is the pure hinge movement of the mandible about a fixed transverse axis through both condyles (the terminal hinge axis or centric relation axis). During the first 20–25 mm of mouth opening, rotation occurs in the inferior joint compartment. Beyond this, translation (forward sliding of the condyle) also occurs in the superior compartment. Rotation = hinge movement about a fixed point. This is the basis of centric relation.