What's the dot in the centre of the red window close button for? When I'm working with certain applications / windows in Mac OS X (I'm using 10.8, although I'm not sure that's relevant), I sometimes see a black dot in the centre of the red window close button. What does this mean? I've seen it most often in Terminal.app, although I think it also happens in other applications too.

I'm talking about this:



versus the normal state of this:
Specifically, it means that hitting that button will produce a prompt rather than immediately performing the action of closing the window. While this is usually a save dialog, it could be anything requiring further user action.

In this sense, it is equivalent to ellipses on the ends of menu entries.