The substance or substances that in the course of a chemical reaction are converted to one or more products.
When a reactant is acted upon by an enzyme the terms substrate instead is used. See forward reaction for some discussion of use of the term reactant with regard to reversible reactions. Note that the term "reagent" can also be used synonymously with reactant.
Note that often in biological reactions not all reactants or, for that matter, products may be shown in a chemical reaction particularly when discussed less formally. Thus, for example, when considering the actions of a hydrolytic enzyme such as a protease, not just the protein or polypeptide involved is a "reactant" (strictly, a substrate) but so too is the water molecule that is inserted into the bond that is being broken. When describing the overall reaction, in shorthand, however, that water molecule often is either ignored as a given or, indeed, simply ignored.
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