Fox is a brevity code used by NATO pilots to signal the simulated or actual release of an air-to-air munition or other combat function. Army aviation elements may use a different nomenclature, as the nature of helicopter-fired weapons is almost always air-to-surface. 'Fox' is short for 'foxtrot', the phonetic designation for the letter 'F', which is short for 'fire'. The radio call announcing that a weapon has been fired is intended to help avoid friendly fire, cuing other pilots to ensure that they do not maneuver into the path of harm.
There are four variations of the Fox brevity word, with a number added to the end of Fox to describe the primary type of sensors the launched munition possesses (if applicable). In addition, a separate brevity word for gunfire exists and is grouped with the four Fox codes for convenience.