Use this tag only if you are creating or modifying a game engine framework, or if the game engine you use doesn't yet have its own tag on Stack Overflow. Otherwise use the engine-specific tag(s).
A game engine is a software framework designed for the creation and development of video games.
Some game engines even include a suite of visual development tools in addition to reusable software components and an asset pipeline that takes care of pre- and postprocessing resource files.
Many so-called "game engines" only provide real-time rendering capabilities instead of the wide range of functionality (physics, collision detection, pathfinding, user interface controls, AI, etc) needed by games. These engines rely upon the game developer to implement the rest of this functionality and are more correctly referred to as a "graphics engine" or "rendering engine".