The Collaborative International Dictionary of English v.0.48:

Ricinus \Ric"i*nus\, n. [L., the castor-oil plant.] (Bot.)
   A genus of plants of the Spurge family, containing but one
   species (Ricinus communis), the castor-oil plant. The fruit
   is three-celled, and contains three large seeds from which
   castor oil is expressed. See Palma Christi.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Ricinus
    n 1: a genus of herb having only one known species: castor-oil
         plant [syn: Ricinus, genus Ricinus]