Media Access Control
From MTU LUG wiki
The Media Access Control address, commonly called the Hardware Address or Physical Address, is the layer 2 address used in Ethernet to uniquely identify a network node.
The name is a misnomer; modern cards and drivers allow the MAC address to be easily altered, so use as a security measure is limited. In the past, network access and software licenses were restricted based on this address.
(for more information, see Wikipedia:Media_Access_Contrl).