International Mobile Equipment Identifier (IMEI) is a code uniquely identifying the a GSM cell phone on the network generally displayed on a phone beneath the battery. They can have 15 (14 decimal digits plus a check digit) or 16 (IMEISV) digits and encoded in the number are the origin, model, and serial number of the device.  Devices generally report their IMEI number by typing *#06# on the phone.