A smart card that serves as a security mechanism in GSM cellular radio networks.The SIM, which plugs into a card slot in the GSM handset, stores user-profile data, a description of access privileges and features, and identification of the cellular carrier that hosts the home registry.The SIM also can store telephone numbers entered by the user into a personal directory.The SIM can be used with any GSM set, thereby providing complete mobility across nations and carriers supporting GSM, assuming that cross-billing relationships are in place. See also carrier, cellular radio, and GSM. SIMPLE (Session Initiation Protocol [SIP] for Instant Messaging and Presence Leveraging Extensions) The predominant IM standard. SIMPLE is an extension of SIP (RFC 3261) to address the suite of services the IETF refers to as Instant Messaging and Presence (IMP). SIMPLE is compliant with IETF specifications for Instant Messaging/Presence Protocol Requirements (RFC 2779) and the Common Profile for Instant Messaging (CPIM) specification (RFC 3860). See also CPIM, IETF, presence, protocol, and SIP.