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.

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

(botany, entomology) A sting on the body of a plant or insect.