A base year refers to the base point in time of a time series such as with a GDP deflator to convert GDP at current market prices into GDP at constant prices.
The reliability of GDP data is distorted by the failure of countries to regularly update their base years.
When GDP is revised and the base year is updated, it allows the statistician to reweight the relative importance of the different sectors of economic activity, and further change or reconsider the methods and data sources.
Data source: World Economics Research, London