The fox one, fox two, fox three terminology is used in voice communications, among American military pilots, to indicate that an air-to-air missile has been launched. To wit:
The launch of an unidentified missile (this is very nearly always a Bad Thing) is signalled simply by "fox."
Therefore, if you're cruising in your F/A-18 at angels one-niner, and your comm radio squalks "Eagle one! Fox three! Break up!" then you'd best yank back on that control stick if you don't want to screw the pooch.
Information on vox commands courtesy of http://tailslide.firelight.dynip.com/vox.asp.