stimulus

noun3

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.

   • From the beginning of the show to the end, vanity is the sole stimulus and reward of action—vanity, that never looks beyond the present. — 1834
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.