This is the average growth rate in real GDP over a period of time.
Below is an illustration of the theoretical growth rate for an economy in its economic cycle. This the level of GDP growth that a country should be achieveing if the peak and trough phases of the business cycle have been averaged out.
Between 1948 and 2012 the UK economy has achieved trend growth of 2.6% p.a. compound. The highest annual rate of growth was 7.4% in 1973 and the lowest was -5.2% in 2009.