HUD head-up display: provides primary flight guidance by projecting information on to a transparent combiner in the pilot's field of view. EASA uses the term head-up display landing system (HUDLS) to describe a system enabling manually flown Category 2 and 3A approaches.
EVS enhanced vision system: a generic term encompassing uncooled systems for head-down situational awareness only and cooled systems for head-up approach guidance.
EFVS enhanced flight vision system: coined by the FAA to describe a system that provides flight guidance in low visibility by superimposing sensor imagery of the outside world on a HUD. Sensors are not limited to infrared, but can include millimetre-wave radar and others. EASA and ICAO will simply use EVS.
SVS synthetic vision system: defined by the FAA as a computer-generated image of the external scene from the perspective of the flightdeck and derived from the aircraft's altitude, precise position and a database of terrain, obstacles and cultural features.
E/SVS? work is under way on combining enhanced and synthetic vision, but there is no generally accepted term. Approaches include inserting the EVS image into the SVS scene as a window, or fusing sensor and synthetic data into a single picture.