The Collaborative International Dictionary of English v.0.48:

Pitahaya \Pit`a*ha"ya\, n. [Sp., prob. from the native name.]
   (Bot.)
   A cactaceous shrub (Cereus Pitajaya) of tropical America,
   which yields a delicious fruit.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pitahaya
    n 1: cactus of the southwestern United States and northern
         Mexico having edible juicy fruit [syn: pitahaya cactus,
         pitahaya, Acanthocereus tetragonus, Acanthocereus
         pentagonus]
    2: highly colored edible fruit of pitahaya cactus having bright
       red juice; often as large as a peach