Mechanical Code…
A unit, based upon temperature difference and time, used in estimating fuel consumption and specifying nominal
annual heating load of a building. For any one day when the mean temperature is less than 65°F (1s0e), there exist as many degree days as there are Fahrenheit degrees difference in temperature between mean temperature for the day and 65°F (1s0e).
(Uniform Mechanical Code® 210.0)