MAC address
The hardware address on your network card — fixed (mostly) and unique to the device.
Different from an IP address which is assigned dynamically: the MAC address is burned into the card at manufacturing, the IP can change every time you restart the router.
Looks like 00:1A:2B:3C:4D:5E. Used on the local network — the moment the packet leaves your router, the MAC is swapped, but the IP stays.