From Statistics Explained
The population figure of a given area is the total number of people in that area at a given time.
Eurostat collects from the EU-27's Member States data on population on 1st January. The recomended definition is the 'usual resident population' and represents the number of inhabitants of a given area on 1st January of the year in question (or, in some cases, on 31st December of the previous year). The population can be based on data from the most recent census adjusted by the components of population change produced since the last census, or based on population registers.
The average population is calculated as the arithmetic mean of the population on 1st January of two consecutive years. The average population is further used in the calculation of demographic indicators, like the crude rates per 1 000 inhabitants, and for some 'per capita' indicators.