Psoriasis: A reddish, scaly rash often located over the surfaces of the elbows, knees, scalp, and around or in the ears, navel, genitals or buttocks. Psoriasis is an autoimmune disease that is mediated by T lymphocytes. It is also a very common disease, Chronic plaque psoriasis affects approximately 2% of people around the world. About 10% to 15% of patients with psoriasis develop joint inflammation (inflammatory arthritis). Treatment options include topical steroid creams, topical vitamin D derivatives, other medications, injections of biologic agents, and exposure to ultraviolet light.