emergency exit (thing)

Some doors, windows, or other openings are designated as emergency exits. You go out of them to escape during an emergency. Sometimes they're alarmed; you have to push a bar which triggers an alarm, waits 15 seconds, and unlocks the door. On busses, you can pull a lever and push the window out, and on an airplane, you just open the door (make sure it's armed first) and an exit ramp will unfurl.