A gun is a common name given to a device that fires high-velocity projectiles. The projectile, its caliber, or diameter, usually designated in fractions of an inch or in millimeters, is fired through a hollow tube known as the gun's barrel. Differing from the musket, the modern gun is rifled, excluding smoothbores on tanks, AFVs and some artillery, with a series of grooves spiraling along the barrel, and indeed the first rifles were known as 'rifled guns'.

USS Iowa (BB-61) fires a full broadside during a target exercise near Vieques Island, Puerto Rico, 1 July 1984.The term "gun" is often used synonymously with firearm, but in military usage the term refers only to artillery that fires projectiles at high velocity, such as tank guns, or naval guns (naval guns are never referred to as cannon). A gunner is a member of the team charged with the task of operating and firing a gun. Thus, by military terms, mortars and all hand-held firearms are excluded from this definition. The exception to this is the shotgun, which is hand-held, has a smooth bore and fires a load of shot or a single projectile known as a slug.

The word "gun" is also applied to some more or less vaguely gun-like or gun-shaped tools, such as staple guns and glue guns.

In a gun-type fission weapon the "gun" is part of a nuclear weapon. The "projectile" is fissile material that is fired and captured inside the device. In the case of nuclear artillery it should not be confused with the gun that fires the whole warhead.

At times, the word gun is used to describe the person holding the weapon rather than the weapon itself, as in "a hired gun".