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Definition: 1 something, e.g. an environmental change, that directly influences the activity of living organisms, e.g. by exciting a sensory organ or evoking muscular contraction or glandular secretion Light acts as a stimulus to the brain . 2 something that rouses or incites to activity; an incentive
Source: The Penguin English Dictionary. London: Penguin, 2007. Credo Reference. Web. 29 August 2012.