Reliability

1. The ability of an item to perform a required function under stated conditions for a specified period of time.

2. The probability that a functional unit will perform its required function for a specified interval under stated conditions.

3. The continuous availability of communication services to the general public, and emergency response activities in particular, during normal operating conditions and under emergency circumstances with minimal disruption.

Source: from Federal Standard 1037C and from MIL-STD-188

In psychometrics, reliability is the consistency of a measure, either internally or over repeated measures, see Reliability (psychometric).