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]