The medium machine gun - the MMG - is something of a retronym for a rifle calibre machine gun which can only be fired in a sustained fire mode: generally from a tripod or other raised mount and served by multiple crewmen. These weapons were arguably the first true machine guns and were later renamed heavy machine guns when the light machine gun was introduced before that classification was restricted to weapons of above rifle calibre.
They are designed explicity for the core machinegun duties of area suppression and volume fire and by virtue of their mass and size they tend to be primarily defensive weapons with limited roles in supporting planned attacks and local air defence.
The MMG is either belt or hopper fed and fitted with either a cooling jacket or quick-change barrels to sustain the required volume of fire.
Like the light machine gun dedicated MMG tend to be older designs that have been supplanted in modern arsenals by the more versatile general purpose machine gun which can fulfill both LMG and MMG roles.
Representative examples of the class would include the Vickers Gun, the Maxim 08 (both water cooled) and the M1919 (heavy barrel air-cooled).