The Collaborative International Dictionary of English v.0.48:

Solarium \So*la"ri*um\, n.; pl. Solaria. [L. See Solar, n.]
   1. An apartment freely exposed to the sun; anciently, an
      apartment or inclosure on the roof of a house; in modern
      times, an apartment in a hospital, used as a resort for
      convalescents.
      [1913 Webster]

   2. (Zool.) Any one of several species of handsome marine
      spiral shells of the genus Solarium and allied genera.
      The shell is conical, and usually has a large, deep
      umbilicus exposing the upper whorls. Called also
      perspective shell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

solarium
    n 1: a room enclosed largely with glass and affording exposure
         to the sun [syn: sun parlor, sun parlour, sun porch,
         sunporch, sunroom, sun lounge, solarium]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "solarium":
   best room, drawing room, foreroom, front room, gallery, lanai,
   living room, parlor, patio, piazza, porch, salon, saloon,
   sitting room, sleeping porch, stoop, sun parlor, sun porch,
   sunroom, veranda