From Statistics Explained
Statistics for related categories can be grouped together or aggregated in order to provide a broader picture. Thus, an aggregate is the combination of related categories, usually within a common branch of a hierarchy, to provide information at a broader level to that at which detailed observations are taken.
By way of illustration, national income and price indices are aggregative, in contrast to the income of an individual or the price of a single commodity, which are not.
The aggregating is usually not done by simple addition, but taking account of the relative importance of the different categories, using weights.