1. A chronic skin disease characterized by dry red patches covered with scales; occurs especially on the scalp and ears and genitalia and the skin over bony prominences.
2. A chronic skin disease characterized by circumscribed red patches covered with white scales.
3. A noncontagious inflammatory skin disease characterized by recurring reddish patches covered with silvery scales.
4. The cause of the disease is unknown, but a genetic predisposition is apparent.
5. Etymology: first recorded in 1684, from Late Latin psoriasis, "mange, scurvy", which came from Greek psoriasis, "being itchy"; from psorian, "to have the itch"; from psora, "itch"; related to psen "to rub".

A medical "clarification" for greater "understanding"

A common chronic, squamous dermatosis, marked by exacerbations and remissions and having a polygenic inheritance pattern. The most distinctive histological findings in well developed psoriasis are Munro microabscesses and spongiform pustules.

It is characterised clinically by the presence of rounded, circumscribed, erythematous, dry scaling patches of various sizes, covered by greyish white or silvery white, umbilicated and lamellar scales, which have a predilection for the extensor surfaces, nails, scalp, genitalia and lumbosacral region.

Central clearing and coalescence of the lesions produce a wide variety of clinical configurations, including annular, or circinate, discoid or nummular, figurate and gyrate arrangements.

Here is much more information about psoriasis.