Reliability is the probability that an item will perform its intended function for a specified interval under stated conditions.

A “reliability function” is this same probability expressed as a function of the time period. Thus, reliability relates to the frequency with which failures occur. Here “failure” means “unsatisfactory performance,” usually representing a judgment of an operator or a maintenance person. This does not preclude the possibility of clear-cut failure, such as complete inoperability, in which case judgment really does not enter at all.

References:

1. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84
2. MIL-STD-721, Definitions of Terms for Reliability and Maintainability, 23 Oct 91