Basic Information
Selective fire is a feature which allows the operator of a firearm to switch between semi-automatic action and fully automatic action, usually by flicking a lever known, appropriately enough as the fire selector1. Some selective fire designs offer the opportunity to fire limited bursts (e.g. of three or five rounds) as well as or instead of fully automatic fire (compare the M16A1 - semi or full auto and the M16A2 - semi or three round bursts) and the term could also be justly applied to weapons like the Franchi SPAS shotguns which can be switched between semi-automatic and pump action.
The other common activator for selective fire is a variable pressure trigger - one pressure for semiautomatic and another for fully automatic - these are particularly common in machine guns.
Selective fire is the defining feature of the assault rifle - although it has appeared on other weapons before and since, from pistols to some cannons.