Stimulus

Noun

1. An external phenomenon that has an influence on a system, by triggering or modifying an internal phenomenon; for example, a spur or incentive that drives a person to take action or change behaviour.

   "an economic stimulus"

2. Something external that elicits or influences a physiological or psychological activity or response, or that affects any of the sensory apparatuses.
3. A stinging part on the body of a plant or insect.