The Collaborative International Dictionary of English v.0.48:

Cassia \Cas"sia\ (k[a^]sh"[.a]), n. [L. cassia and casia, Gr.
   kassi`a and kasi`a; of Semitic origin; cf. Heb.
   qets[imac][=a]h, fr. q[=a]tsa' to cut off, to peel off.]
   1. (Bot.) A genus of leguminous plants (herbs, shrubs, or
      trees) of many species, most of which have purgative
      qualities. The leaves of several species furnish the senna
      used in medicine.
      [1913 Webster]

   2. The bark of several species of Cinnamomum grown in
      China, etc.; Chinese cinnamon. It is imported as cassia,
      but commonly sold as cinnamon, from which it differs more
      or less in strength and flavor, and the amount of outer
      bark attached.
      [1913 Webster]

   Note: The medicinal "cassia" (Cassia pulp) is the laxative
         pulp of the pods of a leguminous tree (Cassia fistula
         or Pudding-pipe tree), native in the East Indies but
         naturalized in various tropical countries.
         [1913 Webster]

   Cassia bark, the bark of Cinnamomum cassia, etc. The
      coarser kinds are called Cassia lignea, and are often
      used to adulterate true cinnamon.

   Cassia buds, the dried flower buds of several species of
      cinnamon (Cinnamomum cassia, atc..).

   Cassia oil, oil extracted from cassia bark and cassia buds;
      -- called also oil of cinnamon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Cinnamomum cassia
    n 1: Chinese tree with aromatic bark; yields a less desirable
         cinnamon than Ceylon cinnamon [syn: cassia, cassia-bark
         tree, Cinnamomum cassia]