Originally a substitution reaction, catalyzed by aluminum chloride, in which an alkyl (R–) group or an acyl (RCO–) group replaces a hydrogen atom of an aromatic nucleus. Later, the term came to imply any of a group of electrophilic reactions in which the production of an intermediate cation or positively polarized complex species was catalyzed, either directly or indirectly, by the presence of a Lewis acid or a Brønsted-Lowry acid. The broadened definition embraces some addition, isomerization, elimination, and cracking reactions as well as the synthetically important alkylation or acylation of aromatic systems.