IM Services Identity Module (ISIM) is an application which resides on a mobile devices SIM card (UICC). It performs security procedures such as authenticating the network, and contains user identity and security keys used to encrypt the information transmitted over-the-air.
The ISIM application can co-exist with SIM and USIM on the same UICC making it possible to use the same smartcard in both GSM networks and earlier releases of UMTS.