An aggregate fruit is one which develops from

Aggregate fruits (etaerio) are those fruits that develops from the merger of multicarpellary apocarpus gynoecium (ovary). Individual carpel or pistil develops into fruitlet, these are matured in cluster on a single receptable.
In contrast, a simple fruit develops from one ovary. Aggregate fruit may also be called as accessory fruits in which part of the flower other than the ovary becomes fleshy and form part of the fruit, e.g. raspberry, etc.