I think this gives a better idea of the functions in the dropdown menu. Options are: Lock, Log Out, Switch User. All of which make sense under the name Session. But you are not leaving the device when you switch user, you also might not be leaving the device when you lock it.
The problem is that "session" is jargon that normal people don't understand. "leave" isn't 100% ideal either IMO, but I'm not convinced that "session" is better.
+1, for longer discussion see VDG group group
Yeah, I figured this would need a much larger discussion.