= Substitution reaction
A <substitution reaction> replaces an <atom> or group in a <molecular entity> with another. The replacement can occur through one elementary step or several; the definition specifies the net chemical change rather than a unique mechanism.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Substitution reaction, entry S06078]{ref}
For example, in $R-X+Nu^-\to R-Nu+X^-$, a <nucleophile> replaces a leaving group. Concerted displacement and a pathway through a <carbocation> can have different concentration dependences and <stereochemical> outcomes even when the written products match. The reacting <functional group>, solvent and competing <elimination reactions> help determine which pathway is observed.
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