International Mobile Equipment Identity is a unique number used to identify mobile phones - the device itself, not the subscriber who uses the phone. The number is usually listed on the battery, and can often be found by dialing *#06#. I tried this on my iPhone this evening and it worked.
Actually when *#06# is dialed four different numbers are shown. The EID; the IMEI; the IMEI2; and the MEID. Each number is shown with a bar code. The IMEI and MEID numbers are nearly identical (MEID simply drops the last digit of the IMEI) and both are used to track individual devices. The IMEI2 number is an indication that a device has two SIM cards. With a second SIM card the phone can operate on two different mobile networks.