An International Mobile Subscriber Identity (IMSI) is a unique number associated with all Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) network mobile phone users used for identifying a GSM subscriber.
Each IMSI number has two parts. The initial part is comprised of six digits in the North American standard and five digits in the European standard. It identifies the subscriber's GSM network operator in a specific country. The second part is allocated by the network operator to uniquely identify the subscriber.
The IMSI is stored in the Subscriber Identity Module (SIM) inside the phone and is sent by the phone to the appropriate mobile network.
The IMSI is an important part of the Home Location Register (HLR) lookup process as it is used to identify to which network a mobile number belongs to. This is why the process is sometimes called IMSI Lookup in addition to HLR Lookup or Number Lookup.