1. an organism’s activities in response to external or internal stimuli, including objectively observable activities, introspectively observable activities (see covert behavior), and nonconscious processes.

2. more restrictively, any action or function that can be objectively observed or measured in response to controlled stimuli. Historically, behaviorists contrasted objective behavior with mental activities, which were considered subjective and thus unsuitable for scientific study. See behaviorism. —behavioral adj.