UK:*UK and possibly other pronunciationsUK and possibly other pronunciations/ˈhiːtweɪv/

WordReference Random House Unabridged Dictionary of American English © 2020
heat wave′, 
  1. Meteorologyan air mass of high temperature covering an extended area and moving relatively slowly.
  2. Meteorologya period of abnormally hot and usually humid weather.
  • 1875–80

Collins Concise English Dictionary © HarperCollins Publishers::
heat wave n
  1. a continuous spell of abnormally hot weather
  2. (not in technical use) an extensive slow-moving air mass at a relatively high temperature

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