El término bala se refiere a todo proyectil de metal. Coloquialmente, y si no se requiere precisión terminológica, el término se utiliza para referirse al conjunto completo de cartucho y proyectil, como en la fotografía de la derecha. Covarrubias da la etimología probable, del verbo griego ballo, arrojar, lanzar. En un primer momento, la definición de esta voz era precisa, diciendo proyectil de metal, que tiene dos condiciones esenciales, ser sólido o macizo y de figura esférica. A mediados del siglo XIX, la bala ya no conservaba ninguno de sus antiguos caracteres al adoptar formas diferentes a la esférica. Hoy presenta forma generalmente alargada terminada en una punta cónica o semiesférica.

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A bullet is a projectile propelled by a firearm, sling, slingshot, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration. The word "bullet" is sometimes colloquially used to refer to ammunition in general, or to a cartridge, which is a combination of the bullet, case/shell, powder, and primer. This use of 'bullet', when 'cartridge' is intended, leads to confusion when the components of a cartridge are discussed or intended. Bullets fired from slings, slingshots, and many airguns (including BB guns) travel well below the speed of sound (about 343 m/s or 1126 ft/s in dry air at 20 °C or 68 °F). Low-power handguns have muzzle velocities generally less than the speed of sound (subsonic), while bullets fired from high-power handguns (such as a .44 Magnum) and from rifles have an initial speed faster than the speed of sound, meaning they are supersonic and thus can travel a substantial distance and even hit a target before a nearby observer hears the "bang" of the shot. Bullet speed through air depends on a number of factors such as barometric pressure, humidity, air temperature, and wind speed.

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