The Collaborative International Dictionary of English v.0.48:

Cold wave \Cold" wave"\ (Meteor.)
   In the terminology of the United States Weather Bureau, an
   unusual fall in temperature, to or below the freezing point,
   exceeding 16[deg] in twenty-four hours or 20[deg] in
   thirty-six hours, independent of the diurnal range.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cold wave
    n 1: a wave of unusually cold weather

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "cold wave":
   Afro, bitter weather, bleak weather, coif, coiffure, cold snap,
   cold weather, conk, depth of winter, freeze, freezing weather,
   frost, haircut, hairdo, hairstyle, hard winter, headdress,
   home permanent, natural, permanent, permanent wave, process,
   raw weather, snap, subzero weather, wave, winter, wintry weather,
   wintry wind, zero weather