(SIM or "SIM card") A component, usually in the form of a miniature smart-card, that is theoretically tamper-proof and is used to associate a mobile subscriber with a mobile network subscription. The SIM holds the subscriber's unique MSISDN along with secret information such as a private encryption key and encryption and digital signature algorithms. Most SIMs also contain non-volatile storage for network and device management, contact lists, text messages sent and received, logos and in some cases even small Java programs.