A benign neoplasm arising from the synovial membrane. Examples include the diffuse giant cell tumor of tendon sheath and localized giant cell tumor of tendon sheath. The disease is benign synovial neoplasm (MONDO_0024715). Also known as: benign neoplasm of synovium, benign neoplasm of the synovium, benign synovial tumor, benign synovial tumour, benign synovioma, benign synovium neoplasm, benign tumor of synovium, benign tumor of the synovium.