Fox Code word (slang term) indicating a certain type of air-to-air missile has been fired; the type is designated by number:
"Fox 1" = semi-active radar missile (AIM-7 Sparrow)
"Fox 2" = heat-seeking missile (AIM-9 Sidewinder)
"Fox 3" = active radar missile (AIM-120 AMRAAM)
"Fox 4" = facetious reference to missile shot; for example, if someone claimed to shoot a missile in an impossible situation, he might be accused of shooting a non-existent "top secret" missile requiring a "Fox 4" call. Also jokingly used when an aircraft is brought down by something other than a missile; for example, a midair collision.
Occasionally fighter pilots will simply say "Fox."
http://www.christianfighterpilot.com/fighterpilotspeak.htm#fox