The Collaborative International Dictionary
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.
Wiktionary
n. (context genetics English) The tendency of cross-breeding to produce an animal or plant with a greater hardiness than its parents; hybrid vigour
Wikipedia
Heterosis, hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic.
An offspring exhibits heterosis if its traits are enhanced as a result of mixing the genetic contributions of its parents. These effects can be due to Mendelian or non-Mendelian inheritance.