Identity

Serial number

All devices have a single unique serial number.

You should find this printed on a sticker on your device, along with a QR code that can be scanned.

You might find other unique identifiers exposed in various interfaces.

IMEI number, associated with the mobile device, that identifies the device to the network (Wikipedia (opens in a new tab))
ICCID number, associated with the SIM (Wikipedia (opens in a new tab))
Bluetooth MAC Addresss, associated with the Bluetooth radio (Wikipedia (opens in a new tab))