Stimulus (plural stimuli) is a change in the environment around a person or thing that can feel the change. It is often used in physiology. Stimulus can include feelings like heat, cold, and pain. When a stimulus is applied to something that can feel it, it causes a reflex. For example, if an animal is burned by touching fire, it will react by quickly moving away from it. Because of this, stimulus can be thought of as something done to an object to make it react.
When a sensory nerve and a motor nerve communicate with each other, it is called a nerve stimulus.
A stimulus is often the first component of a homeostatic control system.