1. a behavioral response to an altered, novel, or sudden stimulus, such as turning one’s head toward an unexpected noise. Physiological components of the orienting response have been identified as well, including dilation of pupils and blood vessels and changes in heart rate and electrical resistance of the skin. [described in 1927 by Ivan Pavlov]

2. any response of an organism in relation to the direction of a specific stimulus. See taxis; tropism. Also called orienting reflex.