Cooling degree-days for a given day are the number of degrees Celsius that the mean temperature is above 18 °C. If the temperature is equal to or less than 18 °C, then the number will be zero. For example, a day with a mean temperature of 20.5 °C has 2.5 cooling degree-days; a day with a mean temperature of 15.5 °C has zero cooling degree-days. Cooling degree-days are used primarily to estimate the air-conditioning requirements of buildings.
Government of Canada - https://climate.weather.gc.ca/glossary_e.html