Crunch (or Rules if you want to be more polite and less demeaning) is roleplaying gamer slang for the actual game mechanics of a system or setting, probably in reference to number-crunching. It is the opposite of Fluff, which describes things in an in-universe sense. For example, a special ability or feat's fluff might say that "You grew up in a school for mages, and have thus learned some of the rudiments of magic"; the associated crunch could be a +2 bonus to Spellcraft and Use Magical Device checks.
One of the traditional failings of powergamers is a focus on crunch to the exclusion of the fluff, creating characters that are mechanically powerful but make little sense from an in-character perspective.
Crunch is a frequently-derided part of all gaming, and it's easy to see why: while plot holes and fluff failures can be ignored or excused away as author choices or stylistic differences, crunch issues are literal game-breakers. You might be able to put Matt Ward out of your mind while playing against 5e Necrons, or gloss over the explanation for why your bishop can move an unlimited number of spaces diagonally, but 7e 40K's challenges were always a little weird and nobody can enjoy a game of Monopoly.
For all of its detractors, though, crunch is the beating heart of all gaming. The rules and mechanics of gameplay are the reason we indulge this hobby instead of telling stories around campfires. Chess, 40k, DnD, all of your memorable moments in these games are powered directly by the rules of the world. If done right the crunch of a game gives its players agency and engagement, which can lead to anything between lighthearted fun and teeth-grinding terror.
Just like Fluff, crunch can grow into fluff of its own. Even without illustrations or flavor text you can feel the oppressive atmosphere of Arkham Horror, the heroic mien of DnD, or the factional flavor of your 40k army. Likewise, games like XCOM or 40k's your dudes let setting and characterization grow out of the results of the crunch. When these feelings don't match up with the fluff you're told about, that's when we complain (loudly and violently). The overwhelming power of 5e's flyers, for example, felt absurdly out of place in the groundpounding grimdark trenches of 40k (why would anyone build legions of defilers and chaos space marines when Heldrakes exist?), whereas your badass veteran Ranger missing a point-blank shotgun round in XCOM just feels wrong. The other side of this bitter pill is when crunch doesn't match other crunch: we blame greedy model salesmen (GW), lazy engineers (GW), or blatant favoritism (also GW). Has anybody faced a Riptide or Wraithknight without wondering how its rules ever made it past the writer's desk?
Heavily mechanical systems aim to explain a world in terms of rolls and numbers, letting the players and skill interactions organically grow the fluff. Risk, 3.5 DnD, and Skyrim are all very much crunch-first simulations wherein stories can be born of the events of the game, not the other way around. Some games, especially older ones like Poker or Checkers, don't really care about the fluff at all; any story explanation of your hand in Go Fish is entirely your own.
Examples of Crunch Overwhelming Fluff
- League of Legends and the handling of the lore, after Riot Games realized they could safely ignore it and the majority of the playerbase would not quit, or would rather watch esport athletes score skillshots than follow the lore. Riot decided to abandon the Institute of War and almost several years of story to focus on the esports scene. Cue butchery of characters on par with Matt Ward.
- In approximately every RPG there's a lull before the final boss fight when the player decides it's time to hit up all of the side quests. Never mind that the dragon god will destroy the world in two days time, or that the evil empire has conquered your city, you've got a backlog of chocobo races and dating minigames to play first (or in one memorable case, a hundred lightning strikes to stand around waiting to dodge). It doesn't matter that their own explanation may make no sense, there's XP and late-game weapons waiting!
- Most modern shooters include a melee attack of some sort. It may be a swipe with a knife or a strike with the butt of a gun, but it's normally very quick and extremely powerful, frequently killing a foe in less time than unloading an entire magazine into their torso. This is done to balance the game, sacrificing some verisimilitude to expand the depth and breadth of the positioning and movement mechanics. No, the developers don't really believe hitting somebody with a sharpened stick is comparable to emptying an assault rifle at them point blank, we promise.