A compartment of the knee is an area of articulation - the knee joint cavity is deemed to have three 'compartments'.
The three compartments of the knee
The three compartments of the knee are:
Unloader Braces for Medial Compartment Knee Osteoarthritis - Implications on Mediating Progression. Ramsey DK, and Russell ME. Sports Health. 2009 Sep; 1(5): 416–426.