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]