Source : Webster's Revised Unabridged Dictionary (1913)

Pityriasis \Pit`y*ri"a*sis\, n. (Veter.)
   A disease of domestic animals characterized by dry epithelial
   scales, and due to digestive disturbances and alteration of
   the function of the sebaceous glands.

Pityriasis \Pit`y*ri"a*sis\, n. [NL., fr. Gr. ?, fr. ?, lit.,
   bran.] (Med.)
   A superficial affection of the skin, characterized by
   irregular patches of thin scales which are shed in branlike
   particles.

   {Pityriasis versicolor} [NL.] (Med.), a parasitic disease of
      the skin, characterized by the development of reddish or
      brownish patches.

Source : WordNet®

pityriasis
     n : any of several skin disorders characterized by shedding dry
         flakes of skin