The Collaborative International Dictionary of English v.0.48:

barrel cactus \bar"rel cac"tus\ (b[a^]r"r[e^]l k[a^]k"t[u^]s),
   n.; pl. barrel cacti (b[a^]r"r[e^]l k[a^]k"t[imac]).
   any of several large cacti native to the southwestern U. S.
   and Mexico, having a short cylindrical form with deep
   vertical ribs and bearing sharp spines. They are classed in
   the genera Ferocactus and Echinocactus.
   [PJC] Barreled

WordNet (r) 3.0 (2006):

barrel cactus
    n 1: a cactus of the genus Ferocactus: unbranched barrel-shaped
         cactus having deep ribs with numerous spines and usually
         large funnel-shaped flowers followed by dry fruits
    2: any cactus of the genus Echinocactus; strongly ribbed and
       very spiny; southwestern United States to Brazil [syn:
       echinocactus, barrel cactus]