CCDA: Cisco Certified Design Associate Study Guide, 2nd Edition (640-861)

Quality of Service: A set of metrics used to measure the quality of transmission and service availability of any given transmission system.

Broadly, any list of elements arranged in an orderly fashion and ready for processing, such as a line of people waiting to enter a movie theater. In routing, it refers to a backlog of information packets waiting in line to be transmitted over a router interface.