This is a code stored on a SIM card that identify the customer.
International Mobile Subscriber Identity
International Mobile Station Identifier: A number assigned to a mobile station by the wireless carrier uniquely identifying the mobile station nationally and internationally. See also MIN, TMSI.
International Mobile Subscriber Identifier. A unique 15-digit number which designates the subscriber. This number is used for provisioning in network elements.
IMSI or Intracytoplasmic Morphologically Selected Sperm Injection is a relatively new technique used by our fertility doctors to treat male infertility.
http://www.themedvacation.com/treating_male_infertility_with_imsi