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:

• Inter-rater (or inter-observer) reliability - The degree of agreement between the results when two or more observers administer the instrument on the same subject under the same conditions. 
• Intra-rater (or intra-observer) reliability - Also known as test-retest reliability, this describes the agreement between results when the instrument is used by the same observer on two or more occasions (under the same conditions and in the same test population). 
• Inter-method reliability - This is the degree to which two or more instruments, that are used to measure the same thing, agree on the result. This is also known as equivalence. 
• Internal consistency reliability - This is the degree of agreement, or consistency, between different parts of a single instrument. (Healthknowledge.org.uk)