1.nounAny external phenomenon that has an influence on a system, by triggering or modifying an internal phenomenon.
"an economic stimulus"
2.nounSomething external that elicits or influences a physiological or psychological activity or response.
3.nounAnything effectively impinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body.
4.nounAnything that induces a person to take action.