As defined by the WMO, this is the average temperature during the 30 years from 1971 to 2000.
1. The WMO recommends that this be recalculated at the end of every decade, so the next redefinition is scheduled for 2011. After that the term will then denote the average temperature from 1981 to 2010.
Example Usages -
1. "…in the strictest sense, a normal of a particular variable (e.g., temperature) is defined as the 30-year average. For example, the minimum temperature normal in January for a station in Chicago, Illinois, would be computed by taking the average of the 30 January values of monthly-averaged minimum temperatures from 1981 to 2010."
(Source: NOAA's 1981-2010 Climate Normals )