IMEI (International Mobile Equipment Identity) — Definition

The IMEI, or International Mobile Equipment Identity, is a unique 15-digit number that identifies your specific phone hardware.

Every phone has one, and it stays with the device no matter which SIM you put in. Carriers use it to check whether a phone is compatible with their network and whether it's been reported lost or stolen.

In practice

You'll need your IMEI when activating service on a phone you bought elsewhere, or when checking if an imported phone will work on a US carrier. Dial *#06# on any phone to see it instantly, or find it in your settings under About.