The Collaborative International Dictionary of English v.0.48:

Nopal \No"pal\, n. [Mexican nopalli.] (Bot.)
   A cactaceous plant (Nopalea cochinellifera), originally
   Mexican, on which the cochineal insect feeds, and from which
   it is collected. The name is sometimes given to other species
   of Cactaceae.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nopal
    n 1: cactus having yellow flowers and purple fruits [syn:
         nopal, Opuntia lindheimeri]
    2: any of several cacti of the genus Nopalea resembling prickly
       pears