synovial membrane (synovium) n. the membrane that forms the sac enclosing a freely movable joint (see diarthrosis). It secretes the lubricating synovial fluid.

synovial membrane The membrane that lines the ligament surrounding a freely movable joint (such as that at the hip or elbow). It secretes a fluid (synovial fluid) that lubricates the layers of cartilage forming the articulating surfaces of the joint.