HUD stands for Head-Up Display, an electronic device that gives the pilot all the needed information without having to bury the eyes inside the cockpit. By that mean, the pilot can track his target with more efficiency.
Picture taken during a 2+2 CAP (Combat Air Patrol), AP (autopilot) selected, 27900 ft altitude, 321 IAS (Indicated Air Speed), Mach 0.81, heading 323°.
Over cloudy France, 2008.[link]