IMSI (International Mobile Subscriber Identifier) is a number of 14-15 digits which identifies a mobile subscriber by their SIM card. It is made up of several parts, including a country code, a network code, and an individual string of digits identifying each particular card within the mobile network.

This unique number is stored on the SIM card itself and is not moved or changed in the case that the mobile phone number (the MSISDN) is ported to a different SIM card. This allows companies to check the IMSI of a subscriber, to cut down on SIM swap fraud.