ICCID (Integrated Circuit Card Identifier) — Definition

The ICCID, or Integrated Circuit Card Identifier, is the unique serial number that identifies a SIM card or eSIM profile.

Where the IMEI identifies the phone, the ICCID identifies the SIM. It's the long number printed on the body of a physical SIM card, and it's assigned to an eSIM when its profile is created.

In practice

Carriers ask for the ICCID when activating a SIM or transferring a number, since it tells them exactly which card to switch on. On most phones you can find it in settings under About or SIM status if the card itself isn't handy.