A stimulus appears and the organism acts. A response is that reaction—muscular, glandular, or neural—elicited or emitted in relation to stimulation.

In classical conditioning the response is elicited by the conditioned stimulus. In operant conditioning the response is emitted and then brought under the control of its consequences. Response measures include latency, amplitude, frequency, and duration. Complex performances are built from simpler responses through shaping and chaining.

The same term covers everything from a spinal reflex to a carefully considered verbal answer. Distinguishing automatic from controlled responses remains important in cognitive and social psychology. Recording and analyzing responses provides the empirical foundation for most behavioral research.

Without measurable responses, psychological claims could not be tested.