The Collaborative International Dictionary of English v.0.48:

Cassava \Cas"sa*va\ (k[a^]s"s[.a]*v[.a]), n. [F. cassave, Sp.
   cazabe, fr. kasabi, in the language of Haiti.]
   1. (Bot.) A shrubby euphorbiaceous plant of the genus
      Manihot, with fleshy rootstocks yielding an edible
      starch; -- called also manioc.
      [1913 Webster]

   Note: There are two species, bitter and sweet, from which the
         cassava of commerce is prepared in the West Indies,
         tropical America, and Africa. The bitter (Manihot
         utilissima) is the more important; this has a
         poisonous sap, but by grating, pressing, and baking the
         root the poisonous qualities are removed. The sweet
         (Manihot Aipi) is used as a table vegetable.
         [1913 Webster]

   2. A nutritious starch obtained from the rootstocks of the
      cassava plant, used as food and in making tapioca.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cassava
    n 1: a starch made by leaching and drying the root of the
         cassava plant; the source of tapioca; a staple food in the
         tropics [syn: cassava, cassava starch, manioc,
         manioca]
    2: cassava root eaten as a staple food after drying and
       leaching; source of tapioca [syn: cassava, manioc]
    3: any of several plants of the genus Manihot having fleshy
       roots yielding a nutritious starch [syn: cassava, casava]