Source : Webster's Revised Unabridged Dictionary (1913)

Psoriasis \Pso*ri"a*sis\, n. [NL., fr. Gr. ?, fr. ? psora.]
   (Med.)
   (a) The state of being affected with psora. [Obs.]
   (b) A cutaneous disease, characterized by imbricated silvery
       scales, affecting only the superficial layers of the
       skin.

Source : WordNet®

psoriasis
     n : 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