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.
In most apps, it's an indicator that there's an open file with unsaved changes (although this is less common now that many apps use OS X auto save). In Terminal it indicates a process is running in the window.
I like to think that it's a gentle remainder: "Something is going on here, and if you close this window, whatever is going on here will be interrupted or lost".
On most apps, it means that the file has been updated (it's "dirty"), and that you will receive a prompt asking if you want to save it or not. Talking specifically about the Terminal, it's telling you that there's a task running, and closing the window will kill the task.
If you have the texteditor open and your file isn't saved, then does it appear. Press cmd+save to save.
