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Definition: A substance used to react with another. Sometimes the term is used synonymously with 'reactant', but it can also specify a more general behaviour, as for example when citing lithium tetrahydridoaluminate as a reagent used to reduce compounds containing the carbonyl group.
Source: The Penguin Dictionary of Science. London: Penguin, 2009. Credo Reference. Web. 10 August 2012.