What is the definition or description of: psoriasis?

Autoimmune, itchy: Psoriasis is an autoimmune disease involving the skin, nails, and occasionally the joints. It is not contagious. There are several types of skin lesions, most common variety being large red scaly itchy plaques on extensor surfaces such as elbows and knees. Psoriasis can be controlled by a wide variety of medications, but a cure has not yet been found.