Definition:
A system’s reliability is a measure of stability and overall performance of a system collated throughout an extended amount of your time under various specific sets of test conditions. this kind of testing incorporates the results from non-functional testing such as stress testing, security testing, network testing, along with functional testing. it’s a combined metric to define a system’s overall dependableness. A measure of reliability should be outlined by business needs within the style of service levels. These needs should then be used to measure test results and also the overall reliability metric of a system under test.
Further Reading:
Book: Testing Practitioner Hanbook by Renu Rajani.