Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Castor-oil plant}. Same as {Palma Christi}.

Source : WordNet®

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