What does the 'S' stand for in the abbreviation SIM, as in SIM card?

The "S" in the abbreviation SIM, which stands for Subscriber Identity Module, represents the term "Subscriber." This is because a SIM card is used by a subscriber (user) to authenticate their identity and establish a connection with a mobile network. SIM cards contain unique information that enables subscribers to access network services, make calls, send messages, and use data on their mobile devices.