The SN1 mechanism is a type of nucleophilic substitution reaction that occurs in two main steps. In the first step, the leaving group departs, forming a positively charged intermediate called a carbocation. This step is slow and determines the reaction rate. In the second step, a nucleophile attacks the carbocation, resulting in the formation of the final product. The SN1 mechanism typically occurs with tertiary substrates due to their ability to stabilize the carbocation. This mechanism is characterized by its unimolecular rate law, meaning the rate depends only on the concentration of the substrate.