Game Engine

In order to understand what a game engine is, it is necessary to take a moment and understand what games are made of. It ain't sugar and spice.

Games are composed of two things: code and data. Code is what makes a game function, and data is what makes it fun. Take Super Mario Bros. as an example. The game code defines the rules of the game. That Mario can jump, run, how fast he runs, that there are monsters, some monsters can be stomped on, etc. The game's data defines what Mario looks like, how he animates, and most importantly the arrangement of terrain and monsters that form levels.

ROM hacking Super Mario Bros. can lead to many Mario-like games. You can replace Mario's sprite, what fireballs look like, and give it a total makeover. But unless you change the code itself, the game will have certain Mario-isms:
 * Fireballs (or whatever you draw in their place) will always go down and forward, bouncing along the ground.
 * Falling off the bottom of the screen will always result in death.
 * Collecting 100 "coins" will always result in an extra life.
 * And so on.

The game will always be to some degree Super Mario Bros.

A game engine is game code that is designed to be data-driven. Unlike the above example, a proper game engine would allow the behavior of in-world entities to be defined in almost every way through data. Virtually all games have some data component, but only relatively recently has this component become flexible enough that two games built from the same engine can be very different from one another. That is, data defines both the function and the fun, while the code is just there to make the data do its job.

One of the first cases of a true game engine was Quake. It was a first-person shooter, but the game engine was much more flexible. It did not even make the assumption that the game was first-person; a user of the engine could pull the camera away to a third-person perspective. And the main character would be rendered there in third person with all of the controls intact and functional. With some work, Quake could have run a game like Super Mario 64, all without directly changing the code of the engine.

Please note that this is a simplification of a complex topic. Many game engines don't quite fit this definition, as the method of customizing an engine often involves writing code in addition to data. Half-Life 2's game engine, the Source engine, is modified by loading DLLs, which are compiled C/C++ code. Even Quake mods, written in so-called QuakeC, were compiled directly into virtual machine assembly code. Scripting throws a wrench into this as well, as scripts are code that is loaded like data. In that case, a game engine is basically just a library or module that handles 80% of the grunt-work in making a game: collision detection, that things can move, rendering things, etc. It is then up to the user to add the 20% that makes the game unique, whether through data and code or purely data.

Also making this more complex are people who sell licenses to engines that involve handing over source code, which is pretty much any commercially available engine. No game developer worth his salt is going to build his game based on your assurance that your engine is bug-free. This allows developers using the engine to actually change the engine code itself, not just layering new code on top of the engine. And developers frequently avail themselves of this opportunity. So a game that claims that they are using the X engine probably made some changes to X.

The general rule of thumb is this: You know it's an engine if you don't have to actually change the engine's core code to make a game that is substantially different game from other games made with this engine.

As a bonus, games that are significantly "enginified" are also very easily modded, and games using the same engine are easy to port if other games using the same engine have already been ported.

Engines can be internal, restricted to a particular development company, or external, which can be licensed for use by others. In-house "engines" are really just common codebases that multiple development teams in a company share. It is impossible to know whether these truly fit the definition of "game engine" because the different teams modify the source code for their own needs. Unless an engine is external, it is difficult to know what you can do with it without modifying the source code.

This may sound like a Game Maker, but there is a difference. A Game Maker is limited to a specific style within a genre. This limitation is what allows them to be easier to use. Game Engines cover a wide range of possible game types. Super Mario World level editing is effectively a Mario-style Game Maker. You can never make anything other than that kind of game. The line between the two, of course, is somewhat fuzzy. And when you touch that fuzzy, it can get dizzying.

External Game Engines

 * SCUMM (Script Creation Utility for Maniac Mansion) by Lucasarts (while it's only possible to make adventure games with it, it's usually considered an engine due to its complexity and wide variety of games): Maniac Mansion (obviously), Monkey Island series (up to Curse), Sam and Max Hit The Road, The Dig, Loom, the Indiana Jones adventure games. Humongous Entertainment used it for every single game they made, such as the Putt-Putt, Freddi Fish, Pajama Sam, Spy Fox, and Backyard Sports series. Went through 11 versions, each one adding more features. Also popular for fanmade games, thanks to ScummVM.
 * id Tech engines by Id Software
 * Wolfenstein 3D engine: Wolfenstein 3D, Rise of the Triad
 * Doom Engine: Doom, Hexen, Heretic, Strife.
 * Quake engine: Quake, Hexen II
 * GoldSrc Engine by Valve, a modified Quake engine: Half-Life, Counter-Strike
 * Xash3D, an open-source engine recreation of GoldSrc allowing its games to be ported to other platforms
 * Quake II Engine: Quake II, Heretic II, Daikatana, Soldier of Fortune
 * id Tech 3 (aka the Quake III Arena Engine): Call of Duty 1, Quake III Arena (and by extension, Quake Live), American McGee's Alice, Return to Castle Wolfenstein, Star Wars: Jedi Knight II - Jedi Outcast, Star Wars: Jedi Knight - Jedi Academy
 * IW Engine by Infinity Ward, as the name implies: used for Call of Duty (from Call of Duty 2 onwards) and the newer James Bond licensed games.
 * ioquake3: an open-source fork of id Tech 3. Used in OpenArena and Urban Terror, among others.
 * id Tech 4 (aka the Doom 3 Engine): Doom 3, Quake IV, Prey, Wolfenstein
 * id Tech 5: Rage, Wolfenstein: The New Order
 * id Tech 6 (aka "id Tech 666"): DOOM (2016), Wolfenstein II: The New Colossus
 * id Tech 7: Doom Eternal
 * The Unreal Engine series by Epic Games (named for the Unreal series, which was the flagship series for it until the more commercially-profitable Gears of War took over the role)
 * Unreal Engine 1: Unreal, Unreal Tournament, Rune, Deus Ex, The Wheel of Time, Clive Barker's Undying. (And if they hadn't been canned, Jazz Jackrabbit 3D and a Hired Guns sequel to the old DOS /Amiga game.)
 * Unreal Engine 2 (and 2.5): Unreal Tournament 2004, Splinter Cell, XIII, Red Steel, Tribes: Vengeance, Red Orchestra: Ostfront 41-45, Killing Floor, Postal 2, etc.
 * Unreal Engine 3 (probably the most popular engine for HD games): Gears of War, Unreal Tournament III, Bulletstorm, BioShock (series), Mass Effect, The Last Remnant, Lost Odyssey, Mirror's Edge, Batman: Arkham Asylum, Batman: Arkham City, Asura's Wrath etc.
 * Also is the subject of a lawsuit over Too Human, charging Epic Games with selling the engine unfinished and giving bad support for developers licensing it.
 * Unreal Engine 4: Fortnite, Unreal Tournament 4, Kingdom Hearts III, Sifu, Star Wars Jedi: Fallen Order
 * Unreal Engine 5: The Matrix Awakens, Ark II
 * Source Engine by Valve: Half-Life 2, Counter-Strike: Source, Left 4 Dead, Portal, Portal 2, Team Fortress 2, Dota 2, Vampire: The Masquerade Bloodlines, Postal 3
 * The Torque Game Engine family by Garage Games: Penny Arcade Adventures, Tribes 2
 * The Lithtech engine by Monolith/Touchdown: Shogo: Mobile Armor Division, No One Lives Forever, Tron 2.0, F.E.A.R., Condemned, Alien vs. Predator 2.
 * The NetImmerse/Gamebryo engine: The Elder Scrolls since The Elder Scrolls III: Morrowind, Warhammer Online, Freedom Force, Fallout 3, Epic Mickey
 * An early example might be Infocom's decades old Z-machine, still in use for Interactive Fiction.
 * And at around the same time, the Freescape engine from Incentive (now Superscape), designed for the implementation of full-3D games even on 8-bit machines. On those it managed about one frame every two seconds, but hey, the fact that it worked at all was amazing back then.
 * The CryENGINE, made by Crytek: Far Cry, Crysis, Aion
 * Game Maker is technically complex enough to be considered a full-fledged 2D Game Engine, not a Game Maker. Ironic.
 * If you know what you're doing, you can even make 3D games with it. Those are much less common, though, and with good reason; They are much harder to do than any 2D game.
 * Build engine (was written by Ken Silverman), which used in Duke Nukem 3D, Shadow Warrior, Blood and a couple of lesser obscure games.
 * BioWare made a habit of making a new engine for each Next Big Thing they bring out (except Mass Effect and Star Wars: The Old Republic). The ealier ones have been licensed to other companies:
 * Infinity Engine (1998-2002): Originally made for Baldur's Gate (and sequel), but licensed to Black Isle Studios for Planescape: Torment and Icewind Dale (and sequel)
 * Aurora Engine (2002-2007): Neverwinter Nights, The Witcher by CD Projekt
 * Electron Engine (2006-ongoing): An offshoot of Aurora, used by Obsidian Entertainment for Neverwinter Nights 2
 * Odyssey Engine (2003-2005): Knights of the Old Republic and its sequel by Obsidian; also, they used a very similar engine for Jade Empire
 * Eclipse Engine (2009-current): Dragon Age: Origins.
 * Mac users might remember the Sprite Animation Toolkit (SAT), which was a complete 2D game engine.
 * BYOND: A 2D tile-based multiplayer game engine. Used by several games with articles on this wiki.
 * Serious Engine by Croteam (named for Serious Sam):
 * Serious Engine 1: Serious Sam (The First Encounter and The Second Encounter), Carnivores: Cityscapes, Deer Hunter 2003, Bird Hunter 2003: Legendary Hunting, Nitro Family, Alpha Black Zero: Intrepid Protocol
 * Serious Engine 2: Serious Sam II
 * Serious Engine 3: The HD remakes of Serious Sam (The First Encounter and The Second Encounter), Serious Sam III: BFE
 * Unity (2006–present) and Cocos2d (2008-present): These are commonly used for making iOS and Android games, and is currently Unreal's chief rival. Both are freely-available for amateur developers, and both offer pre-made assets as a starting point for game development.

Internally Developed Game Engines

 * The 007 Goldeneye engine by Rareware, used in both the game of the same name, and the spiritual successor Perfect Dark.
 * The MT Framework, made by Capcom: Dead Rising, Lost Planet, Devil May Cry 4, Resident Evil 5, Marvel vs. Capcom 3
 * Crystal Tools (originally called the White Engine) by Square Enix: Final Fantasy XIII, Final Fantasy Versus XIII and Final Fantasy XIV. Not to be confused with the Crystal Engine, which was used in Square Enix-published games Tomb Raider: Legend and Deus Ex Human Revolution.
 * Men of War has the GEM Engine, previously used for the prequel Faces of War.
 * The Rockstar Advanced Game Engine (RAGE), which is used in Grand Theft Auto IV, Midnight Club: Los Angeles, and Red Dead Redemption.
 * The Dark Engine created by the now defunct Looking Glass Studios and used in the first two Thief games and System Shock II.
 * Hedgehog Engine by Sega (named for Sonic the Hedgehog, the only series to use this engine so far): An engine built to render objects very quickly. Used in the HD versions of Sonic Unleashed and Sonic Generations.
 * Frosbite by DICE: Battlefield series (from Battlefield 1943 onwards), Need for Speed: The Run, the Medal of Honor reboot's multiplayer. Originally developed with first-person shooters in mind, but has now been the go-to engine by EA's studios, even in sports games such as the FIFA series.
 * UbiArt Framework by Ubisoft: Rayman Origins

Things That Are Commonly Called Engines But Aren't

 * The Havok "Engine". This is not an engine; it is a physics library.
 * Renderware, developed by Criterion Games and bought out by Electronic Arts (notable for making PlayStation 2 development a lot easier). This is not in the most technical sense an engine, because it does not provide a means for using it without modifying the source code. It is a codebase, a large code library that serves as a useful starting point for making games (specifically the renderer hence the name), but it has no inherent extensibility built into it besides writing code. Games made using this codebase include Burnout (Criterion's flagship series), Grand Theft Auto (III, Vice City, and San Andreas), Sonic Heroes and SpongeBob SquarePants: Battle for Bikini Bottom.