night watch

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A watch or guard kept during the night.
• noun The person or persons on such a watch.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a watch kept on some premises, or on a town, by guards during the night
• noun the guards on such a watch

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a watch during the night (as from midnight to 8 a.m.)