Electronic serial number, commonly known as ESN, is a unique identifier used by mobile devices such as smartphones and tablets. This information is used to track and manage device usage, activation, and deactivation. Other synonyms for ESN include device serial number, MEID (Mobile Equipment Identifier), and IMSI (International Mobile Subscriber Identity). Some devices use a different type of identifier called a SIM (Subscriber Identity Module) card, which is a removable card that stores unique information for the device. These terms are all related to the identification and management of mobile devices, and are essential for the proper functioning and security of wireless networks.