The ability to get the same or similar result each time a study is repeated with a different population or group (NICE).
The extent to which a measurement, when repeatedly applied to a given situation consistently produces the same results if the situation does not change between the applications. Reliability can refer to the stability of the measurement over time or to the consistency of the measurement from place to place. (CDC evaluation glossary). (CDC evaluation glossary)
Reliability is the overall consistency of a measure. A highly reliable measure produces similar results under similar conditions so, all things being equal, repeated testing should produce similar results. Reliability is also known as reproducibility or repeatability. There are different means for testing the reliability of an instrument: