The Collaborative International Dictionary of English v.0.48:

Phototherapy \Pho`to*ther"a*py\, n. (Med.)
   The application of light for therapeutic purposes, esp. for
   treating diseases of the skin. -- Pho`to*the*rap"ic,
   Pho`to*ther`a*peu"tic, a.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

phototherapy
    n 1: the use of strong light to treat acne or hyperbilirubinemia
         of the newborn