Definition 5. A windowed operator is to be executed over a certain portion of the data stream (window) as opposed to the whole stream.

Definition 6. A Multimedia Data Stream Management System MMDSMS is a virtual center (could be physically distributed) that receives the tuples of all streams registered within it (both multimedia streams and regular streams). Users use the MMDSMS’s user interface to register a set of CQs and the m-CQs. The MMDSMS responsibility is to continuously process the tuples under real-time constraints with respect to the registered CQs, and to disseminate the results back to the users.