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Meaning of degree-day

de•gree-day

Pronunciation: (di-grē'dā"), [key]
— n. Engin.
  1. one degree of departure, on a single day, of the daily mean temperature from a given standard temperature. Abbr.: dd Cf. cooling degree-day, growing degree-day, heating degree-day.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.