When it comes to fighter jets, the modern world is currently amid a transition. The combat-proven fighters we’ve grown to know and love over the years are reaching the ends of their operational lifespans, and while update programs are underway for many of them, technological strides made in the years since these platforms first took to the skies have finally made it feasible to field a whole new generation of aircraft, more competent and capable than any that came before them.

Or at least, that’s the idea.

These new aircraft fall into the “fifth-generation” of fighter classes fielded by the world’s top militaries. However, what exactly sets them apart from aircraft like the F/A-18 Super Hornet or the F-15 Strike Eagle? At what point does a new design stop being an advancement of a previous generation and become a new generation all it’s own?

Well, the answer is a bit more complicated than you might think. Just like the lines between Generation X, millennial, and whatever other generational names get slapped on folks of certain age groups, the edges of where one generation ends and the next begins start to get blurry the closer they are together. There’s no doubt that the F-16 Fighting Falcon is a fourth generation fighter, but there’s a reasonable debate to be levied about China’s current iteration J-20 despite its general acceptance as a next-gen aircraft. There are some generally accepted conditions an aircraft must meet to be considered a part of the “5th generation,” but what if a plane meets most of them? Or promises to meet all of them later in its operational lifespan?

The reason there’s room for discussion is that these terms aren’t all that official — they’re more like general understandings within the aviation community. By their very nature, these understandings are subject to some debate and discussion, making the generational classifications of fighters a bit more subjective than they may appear. Even organizations that establish hard and true criteria for each generation are only doing so for the sake of simplicity — you can’t force other companies (or nations) to accept your terms of generational division if they don’t agree.

Some companies and publications have even set forth their own criteria for each generation of aircraft, and while they don’t all agree, there are a number of facets of each generation that are widely accepted as generally standard.

Fourth generation fighters are widely considered to be fighters that are capable of sustaining supersonic speeds, utilize some variation in fly-by-wire technology, improved avionics and limited, if any, stealth capabilities. Fourth generation aircraft were designed without stealth in mind, but some have had their radar signatures slightly reduced using things like radar absorbent coatings.

U.S. Air Force F-16 over Iraq | Wikimedia Commons

In the minds of most, it’s stealth that primarily sets fifth-generation fighters apart from their predecessors. Unlike the fourth generation jets that were provided some limited stealth capabilities as an afterthought, evading detection is an intrinsic part of fifth-generation fighter design. America’s F-22 and F-35, the premier 5th generation fighters on the planet currently, were both purpose-built to operate in contested airspace by limiting the means of detection available to opponents — this means not only radar signatures, but reducing IR exhaust signatures and modes of communications had to be considered in the aircraft’s development. As a result of this emphasis on stealth, most fifth-generation fighters utilize internal weapons bays to ensure munitions do not affect the aircraft’s detectability.