BB guns are a type of air gun designed to shoot projectiles named BBs after the birdshot pellet of approximately the same size. These projectiles are usually spherical but can also be pointed; those are usually used for bird hunting. Modern day BB guns usually have a smoothbore barrel, with a bore diameter and caliber of 0.177 inches (4.5 mm). BB for modern day BB guns are usually steel, plated either with zinc or copper to resist corrosion, and measure 0.171 to 0.173 inches (4.34 mm to 4.39 mm) in diameter. Some manufacturers also still make lead balls of slightly larger diameter and which are generally intended for use in rifled BB gun barrels, as were formerly used in BB guns. Some Asian companies make plastic BBs for recreation.

One of the most famous BB guns is the Red Ryder BB Gun by Daisy Outdoor Products, modeled after the western Winchester rifle. First introduced in 1938, the BB gun became an iconic American toy, and is still in production today.

The term "BB" originated from the size of steel ball used in a Shot Shell of the same size. BB shot was nominally 0.180 inches (4.57 mm), but tended to vary considerably in size due to the high allowable tolerances for shotgun shell use. Around 1900, Daisy, one of the earliest makers of BB guns, changed their BB gun bore diameter to .175 inches (4.45 mm), and began to market precision-made lead shot specifically for their BB guns. They called this "air rifle shot", but the "BB" name was already well established, and everyone continued calling the guns "BB guns" and the shot "BB shot" or just "BBs".

A gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol, but there are exceptions, such as the U.S. Air Force's GUU5/P. At one time, land-based artillery tubes were called cannon and sea-based naval cannon were called guns. The term "gun" evolved into a generic term for any tube-launched projectile-firing weapon used by sailors, including boarding parties and marines.

In modern parlance, a gun is a projectile weapon using a hollow, tubular barrel with a closed end — the breech — as the means of directing the projectile (as well as other purposes, for example stabilizing the projectile's trajectory, aiming, as an expansion chamber for propellant, etc.), and firing in a generally flat trajectory.

The term "gun" has also taken on a more generic meaning, by which it has come to refer to any one of a number of trigger-initiated, hand-held, and hand-directed implements, especially with an extending bore, which thereby resemble the class of weapon in either form or concept. Examples of this usage include staple gun, nail gun, glue gun, grease gun. Occasionally, this tendency is ironically reversed, such as the case of the American M3 submachine gun which carries the nickname "Grease Gun".