Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

cold wave
     n : a wave of unusually cold weather