I expect a game engine to consist of
[Edit] after reading Grey's post below I agree that (1) here is not really part of an engine, just a welcome addition.
1) The tools required to build levels including all of the lightmapping/bumpmapping and placement of enemies and objects etc.
2) The code which makes everything happen. i.e. it loads in your levels and everthing just works from the doors opening/closing to the ai of the enemies and the progression from one level to another plus saving/loading and running the game over a network.
A game engine is geared to one genre of game e.g. FPS
You can mod it by adding different textures/sounds/models/levels/music/scripts but you should not need to have to tell it how to render the 3D world or anything as low level as that unless you want to change something fundamental.
[Edit] after reading Grey's post below I agree that (1) here is not really part of an engine, just a welcome addition.
1) The tools required to build levels including all of the lightmapping/bumpmapping and placement of enemies and objects etc.
2) The code which makes everything happen. i.e. it loads in your levels and everthing just works from the doors opening/closing to the ai of the enemies and the progression from one level to another plus saving/loading and running the game over a network.
A game engine is geared to one genre of game e.g. FPS
You can mod it by adding different textures/sounds/models/levels/music/scripts but you should not need to have to tell it how to render the 3D world or anything as low level as that unless you want to change something fundamental.