There's many definitions, but most are variations on the concept of a separatable piece of code. Ie, the code is not intergrated into your game, but is instead written by itself and then used by the application. So someone who usually says they're writing a "graphics engine" is writing the code to handle displaying of models, skyboxes, and handling all the culling, etc. It won't have anything to do with the physics (collision detection, etc) nor the game (handling what happens when the user presses the Fire button in a spaceship). Likewise the physics engine will handle only the physics code, and the sound engine will handle only the sound.

The game engine will use all of these, and incorporate them into a compelete system. It should be (at least, desirably) possible to re-use a graphics engine from game to game. Engines allow for more, (I think the right term is) modular programming. It's how companies can sell an Engine (for example, the Havok Physics Engine) without having anything to do with programming a game.

Of course, this is just one interpretation.

Game Engine Anatomy

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