stimulus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something causing or regarded as causing a response.
• noun An agent, action, or condition that elicits or accelerates a physiological or psychological activity or response.
• noun Something that incites or rouses to action; an incentive.
• noun Government spending designed to generate or increase economic activity.

from The Century Dictionary.

• noun Literally, a goad.
• noun In botany, a sting: as, the nettle is furnished with stimuli.
• noun The point at the end of a crozier, pastoral staff, precentor's staff, or the like.
• noun Something that excites or rouses the mind or spirits; something that incites to action or exertion; an incitement or incentive.
• noun In physiology, something which evokes some functional or trophic reaction in the tissues on which it acts.
• noun In entomology, a stinging-hair.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun A goad; hence, something that rouses the mind or spirits; an incentive.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun this sense?) Anything that may have an impact or influence on a system.
• noun this sense?) (physiology) Something external that elicits or influences a physiological or psychological activity or response.
• noun this sense?) (psychology) Anything effectively impinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body.
• noun this sense?) Anything that induces a person to take action.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any stimulating information or event; acts to arouse action