"Personal Communication System"...

Subscriber ID Module– The subscriber ID module (SIM) is unique to theGSM system. It is a smart card with eight kilobytes ofmemory that can be plugged into any GSM phone.– The SIM contains all subscriber information includingtelephone number (called the International MobileSubscriber Identification (IMIS) in GSM), a list ofnetworks and countries where the user is entitled toservice, and other user-specified information such asmemories and speed dial numbers.– The SIM also offers some protection against fraudulentuse. A GSM phone is useless without a SIM; if the userremoves the card when leaving the phone in a car, forexample, the phone cannot be used unless the thief hasa valid SIM.