Stimulus

The term "stimulus" (plural: stimuli) has several related meanings:
 * In physiology, a stimulus is something external that elicits or influences a physiological or psychological activity or response.
 * In psychology, anything effectively impinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body.
 * In other fields, a stimulus is anything that may have an impact on a system; an input to the system. (E.g., an economic stimulus.)

In most contexts, a stimulus can be described as "stimulating", thereby causing "stimulation" or "over-stimulation".

Although related, the word's meaning is distinct from that of "stimulant."