In an organic reaction the substrate is the molecule that is acted upon by a reagent. In a nucleophilic substitution, for instance, the substrate is the alkyl compound R–X that is attacked by the nucleophile. The term matters because it identifies, among the species present, the one whose structure is being transformed. The substrate is thus distinguished from the reagent (such as the nucleophile or base) that brings about the change.

在有机反应中,底物是指被试剂作用的分子。例如,在亲核取代中,底物是受亲核试剂进攻的烷基化合物 R–X。这个术语之所以重要,是因为它在所涉物种中指明了那个结构正被转化的分子。因此,底物与引起这种变化的试剂(如亲核试剂或碱)相区别。