An International Mobile Equipment Identity is a unique number assigned to every mobile phone, used by cellular networks to keep track of every device. In most countries, changing IMEI numbers is illegal and requires accessing basic functions in a mobile phone's software.
Generally, an IMEI is a 15-digit number. Victims of mobile phone theft can instruct cellular carriers to blacklist a particular IMEI, making use of stolen devices impossible. This is also why it is a crime to change IMEI numbers on a phone; it circumvents the intent of the law to combat mobile device theft. Wiretapping operations can also use IMEI numbers to target specific phones.