The Collaborative International Dictionary of English v.0.48:

Phosphor \Phos"phor\ (f[o^]s"f[^o]r or f[o^]s"f[o^]r), n. [Cf.
   G. phosphor. See Phosphorus.]
   1. Phosphorus. [Obs.] --Addison.
      [1913 Webster]

   2. The planet Venus, when appearing as the morning star;
      Lucifer. [Poetic and Rare] --Pope. --Tennyson.
      [1913 Webster]

   3. A phosphorescent substance. [archaic]
      [PJC]

   4. A substance capable of exhibiting luminescence. This term
      is now most commonly used to refer to substances used in
      fluorescent lights, image intensifiers, or cathode-ray
      tubes (CRT's), which luminesce when excited by ultraviolet
      radiation or an electron beam. In color CRT's, three
      different phosphors, each giving different colors when
      excited, are used.
      [PJC]

WordNet (r) 3.0 (2006):

phosphor
    n 1: a synthetic substance that is fluorescent or
         phosphorescent; used to coat the screens of cathode ray
         tubes