noun

1. Any external phenomenon that has an influence on a system, by triggering or modifying an internal phenomenon.

   “an economic stimulus”

2. Something external that elicits or influences a physiological or psychological activity or response.

3. Anything effectively impinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body.

Synonyms: impetus, impulse, spur, influence