# Reliability Glossary

## Maintainability and Availability Terms

**Availability**

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

**Downtime**

The amount of time during which 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 "__m__ean __t__ime __t__o __r__epair"
and is represented by the mean life value for a distribution of repair
times (*see Maintainability*).

**NHPP**

"NHPP" stands for "__n__on-__h__omogeneous __P__oisson
__p__rocess," 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.

**Uptime**

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