The Collaborative International Dictionary of English v.0.48:

Arthritis \Ar*thri"tis\ ([aum]r*thr[imac]"t[i^]s), n. [L., fr.
   Gr. 'arqri^tis (as if fem. of 'arqri`tis belonging to the
   joints, sc. no`sos disease) gout, fr. 'a`rqron a joint.]
   (Med.)
   Any inflammation of the joints, including the gout. A variety
   of forms of arthritis are recognized, some of which (such as
   rheumatoid arthritis, also called arthritis deformans and
   arthritis nodosa) are chronic and progressive, and lead to
   incapacitation and deformity.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

rheumatoid arthritis
    n 1: a chronic autoimmune disease with inflammation of the
         joints and marked deformities; something (possibly a virus)
         triggers an attack on the synovium by the immune system,
         which releases cytokines that stimulate an inflammatory
         reaction that can lead to the destruction of all components
         of the joint [syn: rheumatoid arthritis, atrophic
         arthritis, rheumatism]