[di-gree-dey] 
| one degree of departure, on a single day, of the daily mean temperature from a given standard temperature. Abbreviation: dd |
| de·gree-day (dĭ-grē'dā') n. A unit of measurement equal to a difference of one degree between the mean outdoor temperature on a certain day and a reference temperature, used in estimating the energy needs for heating or cooling a building. |