Stands for "Media Access Control Address."
A MAC address is a hardware identification number that uniquely identifies each device on a network. Every network interface card, such as an Ethernet card or a Wi-Fi adapter, has a permanent MAC address assigned by its manufacturer; some operating systems allow an adapter's MAC address to be temporarily changed, or "spoofed," in software.
Routers and other network infrastructure equipment use both IP and MAC addresses to track which devices connect to a network; an IP address represents the software connection, while a MAC address represents the hardware connection. A network administrator can configure a network to reject connections from specific MAC addresses or restrict network access to a set of approved MAC addresses.
Since a MAC address uniquely identifies a particular network adapter, there must be enough unique addresses for every device that needs one. The address format uses a 48-bit identifier, formatted as six hexadecimal numbers separated by colons, that provides more than 281 trillion potential addresses. For example, an Ethernet card may have a MAC address of 00:0d:83:b1:c0:8e.
NOTE: When a computer or mobile device connects to a public Wi-Fi access point — or even scans for available networks — its MAC address may be recorded and possibly used to track it as it moves around and connects to other networks. To increase privacy, some operating systems now allow you to automatically create a randomly-generated temporary MAC address when connecting to new Wi-Fi networks.