ICCID stands for Integrated Circuit Card Identifier. It's the unique serial number for a specific SIM — whether physical or eSIM. Where the EID identifies the chip itself, the ICCID identifies the carrier profile loaded onto it.
What it looks like
A 19 or 20 digit number, usually starting with 89 (the industry code for telecoms). Example: 8944501012345678901.
Why it matters
Carriers use the ICCID to bill you correctly, route calls and SMS to the right line, and identify which SIM is connecting from a given device. If you replace a faulty SIM, you get a new ICCID even though your phone number stays the same.