A subscriber identity module (SIM) is a smart card inside of a GSM cellular phone that encrypts voice and data transmissions and stores data about the specific user so that the user can be identified and authenticated to the network supplying the phone service. The SIM also stores data such as personal phone settings specific to the user and phone numbers. A SIM can be moved from one phone to another and/or different SIMs can be inserted into any GSM phone. For example, if a user has one phone but uses it for both personal and business calls, he can change the SIM depending on how he will be using the phone (one card contains his personal identity and data and the second card carries his business identity and data).
A SIM also is referred to as a SIM card.