Arthritis (from Greek arthro-, joint + -itis, inflammation; plural: arthritides) is a group of conditions involving damage to the joints of the body.

There are over 100 different forms of arthritis. The most common form, osteoarthritis (degenerative joint disease) is a result of trauma to the joint, infection of the joint, or age. Other arthritis forms are rheumatoid arthritis, psoriatic arthritis, and autoimmune diseases (autoimmune disorder) in which the body attacks itself. Septic arthritis is caused by joint infection. Gouty arthritis (gout) is caused by deposition of uric acid crystals in the joint, causing inflammation. There is also an uncommon form of gout caused by the formation of rhomboid crystals of calcium pyrophosphate. This gout is known as pseudogout (Calcium pyrophosphate deposition disease).