Reliability is defined as the probability an item will perform the required function under the stated conditions for a stated period time. The required function can be as low as one or two operations through to thousands or millions of operations (as in electrical and electronic components). The stated conditions should include such functions as environmental conditions and loading, speed of operation, possible abuse and overloading.
Talking about reliability, we must also specify what to consider as a 'failure'. As an example, if manufacturing plastic water bottles, leakage of the contents may well be considered a disastrous failure. But the caused of the leakage may range from a moulding fault such as a short shot or hairline bad weld line, to an incorrectly fitting screw cap, due to mismatching of fits or tolerances.
Reliability can also cover functional testing for operational parts or assemblies. This functional test can include static or cyclic pressure testing, operational life-testing and various electrical properties test. Tests for physical properties ranging from abrasion resistance to combustibility may also be design by Reliability Engineers.