A change in the environment is the stimulus; the reaction of the organism to it is the response.

What is a stimulus vs response?

The main difference between stimulus and response is that a stimulus is an event or condition which initiates a response whereas response is the organism's reaction to a stimulus.

What is stimulus-response?

Medical Definition of stimulus-response

: of, relating to, or being a reaction to a stimulus also : representing the activity of an organism as composed of such reactions stimulus-response psychology.