'Heterosis' definitions:

Definition of 'heterosis'

noun

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

From: GCIDE

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]