Repairability is defined as the probability that a failed system will be restored to operable condition in a specified active repair time.

It is useful to express this probability in two forms, the probability density function and the cumulative distribution function. These are called the active repair time density function and the repairability function, respectively. The repairability function expresses the probability that the active repair time does not exceed any given total time.

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