Quality of Service (QoS) refers to the capability of a network to offer a service with a certain quality. The quality of a service is a measure of performance which be defined by a number of different parameters including service response time, loss, signal-to-noise ratio, crosstalk, echo, interrupts, frequency response, loudness levels...etc.
QoS is defined by ITU-T E.800 whihc ca be found here: https://www.itu.int/rec/T-REC-E.800-200809-I/en