stimulus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Something causing or regarded as causing a response.
2. n. An agent, action, or condition that elicits or accelerates a physiological or psychological activity or response.
3. n. Something that incites or rouses to action; an incentive: "Works which were in themselves poor have often proved a stimulus to the imagination” ( W.H. Auden).

Century Dictionary and Cyclopedia

1. n. Literally, a goad.
2. n. In botany, a sting: as, the nettle is furnished with stimuli.
3. n. The point at the end of a crozier, pastoral staff, precentor's staff, or the like. In the staves of ecclesiastical authority the stimulus or point is regarded as the emblem of judgment or punishment.
4. n. Something that excites or rouses the mind or spirits; something that incites to action or exertion; an incitement or incentive.
5. n. In physiology, something which evokes some functional or trophic reaction in the tissues on which it acts.
6. n. In entomology, a stinging-hair.
7. n. The continuation in sense-organ, nerve, and brain of a process of stimulation externally initiated; the physiological excitation which is aroused by the action of an external stimulus.

Wiktionary

1. n. Anything that may have an impact or influence on a system
2. n. Something external that elicits or influences a physiological or psychological activity or response.
3. n. Anything effectively impinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body.
4. n. Anything that induces a person to take action

GNU Webster's 1913

1. n. A goad; hence, something that rouses the mind or spirits; an incentive.
2. n. That which excites or produces a temporary increase of vital action, either in the whole organism or in any of its parts; especially (Physiol.), any substance or agent capable of evoking the activity of a nerve or irritable muscle, or capable of producing an impression upon a sensory organ or more particularly upon its specific end organ.

WordNet 3.0

1. n. any stimulating information or event; acts to arouse action