A piece of software that performs 3D rendering. Often takes advantage of a 3D accelerator to improve fps. Mostly used as part of a 3D game engine to draw a virtual world and all the objects in it.
Some common functions include shading, ray tracing, texture mapping and camera manipulation.
A 3D engine is usually written for computer and video games consoles. When written for a PC or similar hardware (Xbox), it is common to use 3D APIs such as DirectX and OpenGL to provide hardware abstraction (see HAL) allowing the engine to function on as many systems as possible.
It must be noted that 3D engine can be used for more than just games, computer animated movies such as Toy Story and Finding Nemo are composed of pure 3D rendering. Other (non-animated) movies typically feature 3D scenes as well which equally need to be rendered. All this rendering is completed by a 3D engine.
Typical examples of 3D games engines include: the Doom rendering engine, Quake engine, Half-Life engine, the Doom III rendering engine and the Unreal engine. It is common for a 3D game, such as those listed above to release their engines for either public use or for other games developers to use in the creation of new games