from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A signal, usually by siren, that an air raid is over or a danger has passed.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. a signal that danger, such as an air raid, is over; usually a siren at a single pitch
- n. permission to proceed after obstacles have been removed
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a signal (usually a siren) that danger is over
- n. permission to proceed because obstacles have been removed
all + clear (Wiktionary)