Availability
Availability is the probability that an item will be able to function (i.e. not failed or undergoing repair) when called upon to do so. This measure takes into account an item’s reliability (how quickly it fails) and its maintainability (how quickly it can be repaired).

Down time
The amount of time a repairable unit is not operating. This can be due to being in a failed state, administrative delay, waiting for replacement parts to be shipped or undergoing active repair.

Maintainability
The probability that a failed unit will be repaired within a given amount of time. The term is also used to denote the discipline of studying and improving the maintainability of products, primarily by reducing the amount of time required to diagnose and repair failures.

MTTR
MTTR stands for mean time to repair and is represented by the mean life value for a distribution of repair times. (See Maintainability.)

NHPP
NHPP stands for non-homogeneous Poisson process, which is a simple parametric model used to represent events with a non-constant failure recurrence rate. This type of model is often used to model reliability growth and the reliability of repairable units.

Repair
An action that restores a failed part or component to operating condition.

Repair distribution
A mathematical model that describes the probability of repairs occurring over time.

Repairable system
A system that can be restored to operating condition after a failure by the repair or replacement of one or more components.

Up time
The amount of time a repairable unit is operating per design.