Your device's hardware address, or MAC address, is a 12-digit hexadecimal number that is assigned to a device when it is manufactured. It is also commonly described as an "ethernet address" for wired connections or "wifi address" for wireless connections. It can be written without punctuation - e.g. 0123456789ab - or with punctuation as a separator, such as 0123.4567.89ab or 01:23:45:67:89:ab. The capitalization of letters within a hardware address does not matter.
The following articles contain more detailed information on finding your hardware address for specific operating systems and devices:
If you're trying to find the hardware address for a device that isn't listed here, go to your favorite search engine and type in "find hardware address for xyz", where "xyz" is the name of the device.