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”

   “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.”

   “Democrats, meanwhile, point out that Republicans seem to have made a conscious decision, beginning with the stimulus, to oppose anything the president put forward, dooming any chance of renewed cooperation between the parties.”

   “respond to a stimulus”

2. Something external that elicits or influences a physiological or psychological activity or response, or that affects any of the sensory apparatuses.

   “Even light nonpainful stimuli can provoke or exacerbate spontaneous pain; this is not limited to tactile, thermal, or vibratory stimuli, because auditory, visual, olfactory, and visceral stimuli also may be problematic.”

3. A stinging part on the body of a plant or insect.

   “Many plants, like many animals, are furnished with arms for their protection; these are either aculei, prickles […]; or stimuli, stings, as in the nettles, which are armed with a venomous fluid for the annoyance of naked animals.”