A stimulus in psychology is an energy change (such as light or sound) which is received by the senses. The usage changes somewhat according to which school of psychology is using it:

• In classical conditioning and behaviorism, a stimulus is the basis for behavior.
• In perceptual psychology it is the basis for perception.^[1] In this context, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs).^[2]
• In experimental psychology, the term 'stimulus' is used to describe the event or object to which a response is measured. In this case, not everything that is presented to participants is a stimulus. For example, a fixation cross is not said to be a stimulus, because it merely serves to center subject's gaze at the center of the screen. Also, longer events are usually not called 'stimuli', even when a response to such an event is measured.

A stimulus may be used to see if a person is in a coma and what type of coma it is.