What is medium machine gun?

A Medium Machine Gun is a type of machine gun characterized by firing full power rifle rounds(such as 7.62x51 mm NATO) and being usually belt-fed.
While they are light enough to be carried by one man they are usually crew served due to weight of the ammunition.
They can be used by infantry, vehicles and aircraft.
Here are some examples of MMG:M240, MG3 and MG42, FN MAG, M60, PKM.