Any structural anomaly of the synovium, which is a membrane that lines the cavity of synovial joints and consists of a lining layer of macrophage-like synoviocytes and fibroblast-like synoviocytes, as well as a sublining of connective tissue. Synovial cells are thought to contribute to joint homeostasis by secreting various factors such as hyaluronic acid and lubricin important for joint lubrication and function, as well as disposing of the waste products.