cold wave

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An onset of unusually cold weather within a 24-hour period.

Wiktionary

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

GNU Webster's 1913

1. (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.

WordNet 3.0

1. n. a wave of unusually cold weather