Different people and groups use the term Quality of Service in somewhat different ways but, in general, the term is used to imply the general characteristics, and "goodness", of the connection used in a data transmission. These characteristics, and the resulting perception that the connection is "good", include the average number of bits per second that can be constantly passed through the link (throughput), the maximum capacity of the link for handling bursty conditions and for supporting multiple users at the same time (bandwidth), and the delay experienced at the packet level, and at the user level.

The International Telecommunication Union (ITU) standard X.902, Information technology - Open distributed processing - Reference Model, says that QoS is "A set of quality requirements on the collective behavior of one or more objects." A number of QoS parameters describe the speed and reliability of data transmission, e.g., throughput, transit delay, and error rate. In the realm of Asynchronous Transfer Mode (ATM), QoS is defined as "A term which refers to the set of ATM performance parameters that characterize the traffic over a given virtual connection." In RFC 1946, the Internet Engineering Task Force states "As the demand for networked real time services grows, so does the need for shared networks to provide deterministic delivery services. Such deterministic delivery services demand that both the source application and the network infrastructure have capabilities to request, setup, and enforce the delivery of the data. Collectively these services are referred to as bandwidth reservation and Quality of Service (QoS)."