"The extent to which a learner emits untrained responses that are functionally equivalent to the trained target behavior" (Cooper, Heron, & Heward, 2007, p. 620).

"When an antecedent stimulus has a history of evoking a response that has been reinforced in its presence, the same type of behavior tends to be evoked by stimuli that share similar physical properties with the controlling antecedent stimulus" (Cooper, Heron, & Heward, 2007, p. 705).