Outgrowths of synovial membrane composed of villi and fibrous nodules characterized histologically by hemosiderin- and lipid-containing macrophages and multinucleated giant cells. It usually occurs in the knee. The disease or phenotype is pigmented villonodular synovitis (EFO_1001106). Also known as: Diffuse Giant cell tumor of Tenosynovium, Synovitis, Pigmented Villonodular, villous tenosynovitis.