When you talk about what game engine is you also need to take into account of what kinda game you wanna make. For instance:
Text based game game engine would require next:
- keyboard handling to alow player to type in commands
- command parsing so that proper method is executed based on inputed command
- ingame object storage
- ingame event handling
And that is it. A fully capable text based game engine.
For a point and click adventure you would need game engine which would add athleast 2D graphics rendering and some sound support to the above.

You see the definition of when bunch of frameworks become game engine largely depends on what kind of a game you wanna make with it.