The SN1 reaction, or "Substitution Nucleophilic Unimolecular," is a type of chemical reaction where a nucleophile replaces a leaving group in a molecule. This process occurs in two main steps. First, the leaving group departs, creating a positively charged intermediate called a carbocation. This intermediate is then attacked by a nucleophile, which is a species that donates an electron pair. SN1 reactions typically happen with tertiary or some secondary alkyl halides because these structures can stabilize the carbocation formed during the reaction. The rate of the reaction depends only on the concentration of the substrate, making it unimolecular. This characteristic distinguishes it from SN2 reactions, which involve a simultaneous attack by the nucleophile.