SIM (Subscriber Identity Module) — Definition

A SIM, or Subscriber Identity Module, is the card or built-in chip that stores your subscriber identity and connects you to a carrier.

It's what tells the network who you are and which account to bill. The physical version comes in nano, micro and standard sizes; the built-in version is the eSIM. Either way, it holds the identity that links a phone to a phone number and a plan.

In practice

Swapping a SIM into a new phone is one of the most common moments APN settings break, because the new phone may not auto-configure the way the old one did. If that happens, your carrier's APN page has the values to set things right.