The Collaborative International Dictionary of English v.0.48:

Cactus \Cac"tus\, n.; pl. E. Cactuses, Cacti (-t[imac]).
   [L., a kind of cactus, Gr. ??????.] (Bot.)
   Any plant of the order Cactac[ae], as the prickly pear and
   the night-blooming cereus. See Cereus. They usually have
   leafless stems and branches, often beset with clustered
   thorns, and are mostly natives of the warmer parts of
   America.
   [1913 Webster]

   Cactus wren (Zool.), an American wren of the genus
      Campylorhynchus, of several species.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cactus
    n 1: any succulent plant of the family Cactaceae native chiefly
         to arid regions of the New World and usually having spines