1) A chronic systemic disease, primarily of the joints, marked by inflammatory changes in the synovial membranes and articular structures, widespread fibrinoid degeneration of the collagen fibers in mesenchymal tissues, and by atrophy and rarefaction of bony structures. Etiology is unknown, but autoimmune mechanisms have been implicated.

2) A usually chronic disease that is considered an autoimmune disease and is characterized especially by pain, stiffness, inflammation, swelling, and sometimes destruction of joints -- abbreviation RA; called also atrophic arthritis.