A 32-bit number assigned by the mobile station equipment manufacturer, uniquely identifying the mobile station equipment.

A unique unchangeable number that is built into the phone and is transmitted by the phone as a means of identifying itself within the system.

The unique identification number embedded in a wireless phone by the manufacturer. Each time a call is placed, the ESN is automatically transmitted to the base station so the wireless carrier's mobile switching office can check the call's validity. The ESN cannot easily be altered in the field. The ESN differs from the mobile identification number, which is the wireless carrier's identifier for a phone in the network. MINs and ESNs can be electronically checked to help prevent fraud.

Electronic Serial Numbers (ESNs) were created by the FCC to uniquely identify mobile devices from the days of AMPS in the United States in the mid-1980s on. The administrative role was later taken over by TIA in 1997. ESNs are mainly used with AMPS and CDMA phones in the United States, compared to IMEI numbers used for GSM phones in Europe and elsewhere.