Every device that is capable of connecting to a network is attached to a unique physical hardware address. This is called 'Media Access Control address', or simply MAC address.
The MAC address is nothing that easily can be changed, it is meant to be a global unique identifier. However, it is possible on most products today and often referred to as MAC spoofing.
This may allow the bypassing of access control lists on servers or routers, either hiding a computer on a network or allowing it to impersonate another computer.
The MAC address is also known as an Ethernet Hardware Address (EHA), hardware address, adapter address, or physical address.
They are assigned by the producer and managed by the Institute of Electrical and Electronics Engineers (IEEE).