A unique identifier for hardware of common wireless communication protocols
Traditionally, every network card that ships (e.g. WiFi, Bluetooth, Ethernet) has an unchangeable unique identifier called a MAC Address, assigned during manufacturing by the manufacturer. This identifier is broadcast as soon as the network card is connected to a network or attempts to communicate. Such an identifier is needed to route messages correctly on the network.
Due to privacy concerns, major manufacturers (like Apple, Google), have, in recent years, implemented MAC Address Randomization, which periodically changes the MAC Address from the factory value to a different random value.
See also Wikipedia entry on MAC address.