A machine gun designed to be carried and served by one man (although they are often deployed with a Gunner's mate to carry spare barrels and extra ammunition). These are generally full rifle calibre weapons (7-9mm, e.g. 7.62mmNATO) although small rifle calibre weapons (<7mm, e.g. 5.56mmNATO) also appear.
Since they are single manned a light gun is generally fitted with a bipod and a traditional, long arms style stock, albiet with a rear grip (i.e. a grip for the off-hand behind the trigger group). Aside from that, there are a variety of designs, some with quick change barrels, some with heavy barrels instead and pretty much any sight fit you would expect on any other firearm - but they should normally have a belt feed or at the very least a high capacity magazine or they start to look more like a support rifle.
Ironically the true, purebred light machinegun is a pretty rare beast - in generally, ever since the invention of the MG34 the niche has been mainly occupied by General Purpose Machine Guns deployed in the light role … or the same basic models modified to reduce mass at the cost of removing their medium/heavy capability. The most significant LMG currently in service are probably the small calibre ones like the M249/L108 (both essentially the FN Minimi).