Linear Warriors, Quadratic Wizards
Linear Warriors, Quadratic Wizards is a /tg/-related trope that was popularized, if not created, by Dungeons & Dragons. It has its roots in Vancian Casting, and in particular the contrast between the Fighter and the Wizard (or, really, any martial character vs. any spellcaster, but we'll stick with the originals), and is a huge source of skub.
In a nutshell, LWQW refers to the disparity between fighters and wizards caused by the differing returns received as they level up. Fighters advance in a linear fashion; their skills improve, their attacking accuracy increases, and their attacks per round slowly increase. However, wizards advance in a quadratic fashion; they can cast more spells per day, they can cast more potent kinds of spells, and all of their lower-level spells increase in potency passively as well.
The end result? The high level fighter is clearly better than he was when he started... but the equally high-level wizard outclasses him in terms of potency and options.
This is a problem that has been around since Advanced Dungeons & Dragons; one of the key reasons that the gish concept through multiclassing took off so much was because it combined the fighter's low-level survivability with the wizard's high-level potency. It only got worse in Dungeons & Dragons 3rd Edition; the new multiclassing mechanics, combined with the scourge of "Ivory Tower Design" (aka, some choices are deliberately designed to be inferior to others) and the Fighter's front-loaded class design meant it was far more optimal to make it your "dip class" and focus your leveling on a more potent class.
Dungeons & Dragons 4th Edition attempted to end this problem by inventing the AEDU System, which definitely worked, but was enormously skubby.
Dungeons & Dragons 5th Edition also attempted to solve this problem with a less overtly new mechanic. Their solution was to remove the idea of spells automatically increasing in potency due to their caster's level; in this version, you have to sacrifice a spellcasting slot of higher level to increase the might of your spells.