stimulus

1. (physiology, psychology, medicine) Something external that elicits or influences a physiological or psychological activity or response, or that affects any of the sensory apparatuses.Examples: "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."

2. (botany, entomology) A stinging part on the body of a plant or insect.Examples: "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."