What specifically is a game Engine?

When someone talks about a game engine I'm a little fuzzy.
How much of the actual game does the engine cover?

Any links to good articles on game engines would be appreciated.

In truth there are many engines to a game i'll explain those then i will tell you what the real(i guess) "Game Engine" if there even is such a thing.

Some of you may argue that many of these are or are not engines or their grouped wrong or i missed some, thats not the point this is just to give the idea of game composition and what if anything the engine truely is.

there many parts(engines) to a game:

Graphics Engine: Renders graphics,

Input Engine: Handles Game input,

Game Logic And Physics Engine: Usually grouped but they need not be, they handle the Logic(AI, game flow, ect...) and Physics(you know what they are),

Particle Engine: Handles the control of Particles(eg. Snow, Fire, Rain, Fog(sometimes),ect...),

Sound Engine: Handles sound,

Game Inteface Engine: Handles Game Interface ,Menuing, User Interface,

etc,etc...

so the "Real Engine" is what brings these all together, and allows them to communicate and interact.

So, basiclly a "Game Engine" is an interface for all the other engines between one another.

edit: let me rephrase that,

A "Game Engine" is an interface between all the other engines, and a control for all the other engines, sort of a master engine for which all the other engines work...

Ok, recently I've heard that the quake 2 engine source is now availble, what is missing that the game would need to run?

models,maps,weapons,textures,ect... all the resources other than the source.