Stimulus (psychology)
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.