IMEI (International Mobile Equipment Identity) - definition
IMEI is a unique 15-digit serial number that uniquely identifies a GSM or UMTS mobile phone. It consists of four parts and provides information, such as the manufacturer, to the mobile network. It is usually printed on the device under the battery.
IMEI can be used to "ban" a stolen phone, making it hard to use by the thief with a new SIM card.