degree day

Short definition from Wiktionary

Noun

1. 1
   A measure of heating or cooling, computed as the integral of a function of time that generally varies with temperature.
2. 2
   A unit used in estimating the fuel consumption for a building; equal to the number of degrees that the mean temperature, for a 24-hour day, is below the base temperature, often 65F.
3. 3
   Alternative form of degree day.