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 precision (i.e., freedom from error) of the scores of a measure, see Reliability (psychometric).