Final answer:
The condyle of the mandible fits into the glenoid fossa with the articular disc separating them, forming the temporomandibular joint which allows complex jaw movements.
Step-by-step explanation:
The portion of the mandible that fits into the glenoid fossa with the interarticular disc separating them is the condyle of the mandible. This articulation forms the temporomandibular joint (TMJ), which allows for various movements such as elevation, depression, protraction, retraction, and side-to-side motions of the lower jaw.
The articular disc is significant as it serves to smooth the movements between the temporal bone and the mandibular condyle, and is integral in facilitating the complex and gliding motions of the mandible during the opening and closing of the mouth.