Climate is typically defined as the average weather (or more rigorously a statistical description of the average in terms of the mean and variability) over a period of time. These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
In climate modelling, the standard time frame over which climate is measured is 30 years (for example by the World Meteorological Organisation , WMO. In UKCP09 a baseline climate of 1961-1990 is used as a reference climate against which to base projections of future climate change. For example the projected temperature between 2011 and 2040 is expressed in terms of difference from the average temperature between 1961 and 1990.
Further explanation is provided in Box 1.1 of the UKCP09 Climate change projections report.