Heating Degree-Day (HDD) is a measure of the need for heating based upon outdoor temperatures. Heating degree-days are calculated as follows:
Daily mean temperature minus 18.3 °C (65 °F)
For example, if the average temperature for a given day was 50 °F, then that would be a 15 degree day. Heating degree days can be added over periods of time to provide a rough estimate of seasonal heating requirements. For example, the number of heating degree-days in Florida, is likely to be much lower than those in London.
The temperature for calculating the Heating degree-day is culture (and technical) dependent. While the above 65°F are true for the USA, in cold countries like Austria and Switzerland it is 12 °C. Germany had it at 12 °C, raised it to 15 °C and is currently (2013) thinking to reduce it again to 12 °C because of the better isolation of the houses.
See also Cooling Degree-Day (CDD).