Erythrodermic psoriasis: A particularly inflammatory form of psoriasis that often affects most of the body surface. It is the least common form of psoriasis and often appears on people who have unstable plaque psoriasis, where lesions are not clearly defined. The erythrodermic form of psoriasis is characterized by periodic, widespread, fiery redness of the skin. The erythema (reddening) and exfoliation (shedding) of the skin are often accompanied by severe itching and pain. Swelling may also develop.