Quality of Service
Definition 1: The collective effect of service performance which determines the degree of satisfaction of a user of the service (ITU-T Rec. E.800) and the statement of the level of quality required by the applications of consumers/users of a service, which may be expressed non-technically (ITU-T Rec. G.1000).

Definition 2: A measure of network performance that reflects the quality and reliability of a connection. QoS can indicate a data traffic policy that guarantees certain amounts of bandwidth at any given time, or can involve traffic shaping that assigns varying bandwidth to different applications.