The Collaborative International Dictionary of English v.0.48:

Pityriasis \Pit`y*ri"a*sis\, n. [NL., fr. Gr. ?, fr. ?, lit.,
   bran.]
   1. (Med.) A superficial affection of the skin, characterized
      by irregular patches of thin scales which are shed in
      branlike particles.
      [1913 Webster]

   2. (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.
      [Webster 1913 Suppl.]

   Pityriasis versicolor [NL.] (Med.), a parasitic disease of
      the skin, characterized by the development of reddish or
      brownish patches.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pityriasis
    n 1: any of several skin disorders characterized by shedding dry
         flakes of skin