SIM means:

SIM stands for Subscriber Identity Module, the small card that identifies a mobile subscriber on the network.

It securely stores the subscriber number and the details needed to authenticate on the operator's network.

Special Identity Module is not a recognised expansion of the term.

Subscriber Information Module wrongly replaces Identity with Information, which is not the standard name.

System Information Module has nothing to do with a subscriber and is not the real meaning.

A modern SIM can be a tiny physical card or an embedded eSIM programmed directly into the device.