A barbette is a protective circular armour feature around a cannon or heavy artillery gun. The name comes from the French phrase en barbette referring to the practice of firing a field gun over a parapet (defensive wall) rather than through an opening (embrasure). The former gives better angles of fire but less protection. For example, the Confederate defenders at the Second Battle of Fort McAllister were unable to operate their cannons because the en barbette gun emplacements provided poor protection from Union riflemen outside the fort.
Before the complete introduction of the fully enclosed armoured gun turrets, a barbette was a fixed armoured enclosure protecting the gun. The barbette could take the form of a ring of armour around the gun mount over which the guns (possibly fitted with a gun shield) fired.
In warships from the age of the dreadnought forward, the barbette is the non-rotating drum beneath the rotating gun turret (properly known as the "gunhouse") and above the armoured deck on a warship. It forms the protection for the upper ends of the hoists that lift shells and their propelling charges (e.g. cordite) from the magazines below.
When applied to military aircraft, largely in aviation history books written in British English, a barbette is a position on an aircraft where a gun, or guns, are in a mounting which has a restricted arc of fire when compared to a turret, or which is remotely mounted away from the gunner. As such it is frequently used to describe the tail gunner position on bombers such as the B-17 Flying Fortress, with American English aviation books usually describing this position as a "flexible" gun mount, when the term "turret" itself is not used.
The term "barbette" (again, mostly as a convention of British English) can be used to describe a remotely aimed and operated gun turret emplacement on any military aircraft of World War II. The German Messerschmitt Me 210 and Me 410 Hornisse had FDSL (Fernbedienbare Drehseitenlafette) 131 twin remote turrets - one per side - for rearward defense. The Germans themselves always used the generic term "lafette" as an integral part of the nomenclature for describing these remotely controlled weapon mounts, and indeed for almost any movable defensive gun mount in German World War II-era aircraft.