stimulus

Noun

stimulus (plural stimuluses or stimuli)

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.1834, L[etitia] E[lizabeth] L[andon], chapter XV, in Francesca Carrara. […], volume II, London: Richard Bentley, […], (successor to Henry Colburn), →OCLC, page 174

   Synonyms:

   influence, impetus, impulse, spur, trigger

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.2002, Kim Burchiel, Surgical Management of Pain, Thieme, →ISBN, page 44
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.1789, Erasmus Darwin, The Loves of the Plants, J. Johnson, page 15