Reliability is a test’s first requirement and refers to its consistency: a reliable test is one that yields consistent scores when a person takes two alternate forms of the test or when he or she takes the same test on two or more different occasions.
Definition 2.
Reliability is a measure of the probability that a product will not malfunction or fail within a specified time period.
Reliability is the characteristic which refers to the consistency of scores obtained by the same person when retested with the identical or equivalent tests.
Reliability is a statistical term for the internal consistency of a test; the extent to which it can be expected to produce the same result on different occasions.