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

The story behind the answer

S stands for Subscriber in SIM, which expands to Subscriber Identity Module. The SIM identifies the subscriber (the account holder) to the mobile network. It stores your IMSI (International Mobile Subscriber Identity) and cryptographic keys so the network can authenticate you, connect calls/data, and bill the right account—regardless of which phone the SIM is in.

Introduced with GSM in the early 1990s, the removable SIM let users move their service between devices; today, eSIMs do this digitally. Remember it as “your identity, not the phone’s”: SIM = Subscriber Identity Module. That’s why “Secure,” “Single,” or “Solid” are misleading—even though SIMs enable security, their core purpose is identifying the subscriber.