In the programming world, the word aggregate relates to a collection of data objects that form a data type. In other words, an aggregate objectis one which contains other objects. For example, A car consists of several parts, like the engine, transmission, brakes, gearbox, etc.
Sometimes the class aggregation corresponds to physical containment in the model (like the car). But sometimes it is more abstract (e.g. Engine and Brakes).
It is interesting how the word aggregate in the normal sense and from a programming perspective means the same thing.