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