Aggregation has two aspects:

(1) the thematic dimension, e.g. combining several variables into one 'aggregate' or by adding up for example figures on land cover of different hierarchical levels of a classification to an upper level (e.g. from 44 CORINE Land Cover classes on the 3rd level to 15 classes at 2nd level),

(2) the spatial dimension, e.g. adding up (or averaging) figures referenced to small reference units to larger areas (upper levels of the regional reference system) without changing the meaning. (EC Glossary Agriculture).