|a unit used in estimating fuel requirements in heating buildings. It is equal to a fall of temperature of 1 degree below the mean outside temperature (usually taken as 18°C) for one day|
A unit of measurement equal to a difference of one degree between the mean outdoor temperature on a certain day and a reference temperature. The unit is most often used in estimating the energy needs for heating or cooling a building (for example, heating degree-days and cooling degree-days). Originally, degree-days were used to determine the relationship between temperature and plant growth. The term continues to be used in life sciences as a measure of upper- and lower-temperature limits for organisms.