Various features in jet fighters are described in terms of “generations”, whereby a typical jet fighter of a given generation tends to have a certain class of engines, avionics, etc., and a typical jet fighter of the succeeding generation tends to have a different (and superior) set of engines, avionics, etc.
A fighter aircraft is a military aircraft designed primarily for air-to-air combat against other aircraft, as opposed to bombers and attack aircraft, whose main mission is to attack ground targets. The hallmarks of a fighter are its speed, maneuverability, and small size relative to other combat aircraft.
The term generation first appeared in the 1990s, according to the Royal Australian Air Force’s Air Power Development Centre Bulletin: “to make sense of the leap-frogging improvements in performance to jet fighter aircraft brought about through major advances in aircraft design, avionics, and weapon systems”.