The Collaborative International Dictionary of English v.0.48:

Castor oil \Cas"tor oil\ (k[a^]s"t[~e]r oil`).
   A mild cathartic oil, expressed or extracted from the seeds
   of the Ricinus communis, or Palma Christi. When fresh the
   oil is inodorous and insipid.
   [1913 Webster]

   Castor-oil plant. Same as Palma Christi.
      [1913 Webster]

WordNet (r) 3.0 (2006):

castor oil
    n 1: a purgative extracted from the seed of the castor-oil
         plant; used in paint and varnish as well as medically