Hi Muhammmad,

Aggregators are actually new actors in the sector and their role is not yet clearly defined as they does not exist in practice yet (as to my knowledge). However, several projects have used their own definition to these actors. One definition is the one used in ADDRESS project which says :

"An aggregator is seen as a mediator between the consumers and the market, gathering flexibilites and contributions of customers, collecting requests and signals from markets and participants, with different levels of optimization to meet the requirement of topologically dependent services."

You can find some reports of the project on http://www.addressfp7.org/

Hope this helps!