What is the specific name which the change or reaction an organism has to a stimulus?

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

Explanation:

In physiology, a stimulus (plural stimuli or stimuluses) [1] is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction. These sensory receptors can receive information from outside

Tropism is the specific name which the change or reaction an organism has to a stimulus