The Collaborative International Dictionary of English v.0.48:

Heterosis \Het`e*ro"sis\, n. [NL., fr. Gr. ? alteration, fr. ?
   other, different.] (Rhet.)
   A figure of speech by which one form of a noun, verb, or
   pronoun, and the like, is used for another, as in the
   sentence: "What is life to such as me?" --Aytoun.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heterosis
    n 1: (genetics) the tendency of a crossbred organism to have
         qualities superior to those of either parent [syn:
         heterosis, hybrid vigor]