cold wave

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

• noun An onset of unusually cold weather within a 24-hour period.

from the GNU version of the Collaborative International Dictionary of English.

• (Meteor.) In the terminology of the United States Weather Bureau, an unusual fall in temperature, to or below the freezing point, exceeding 16° in twenty-four hours or 20° in thirty-six hours, independent of the diurnal range.

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

• noun A short period of very cold weather; a cold snap
• noun A permanent wave in the hair set by special solutions without the aid of any heating machine.

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

• noun a wave of unusually cold weather