A system is linear if its response is directly proportional to excitation, for every part of the system.

Maintenance, corrective

The actions performed, as a result of failure to restore an iten to a specified condition.

Maintenance, preventive

The actions performed in an attempt to retain an item in a specified condition by providing systematic inspection, detection and prevention of incipient failure.

Mass

A physical property, dynamically computed as acceleration/force.. Statistically computed as W (which can be measured on a butcher scale) /g. Ordinary structures are not pure masses as they contain reactive elements, i.e. springs and damping.

Mean Life

The arithmetic mean of the times to failure of a group of nominally identical items.

Mean Time Between Failures (for repairable items)

The product of the number of items and their operating time divided by the total number of failures.

Mean time to repair (MTTR)

The total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time.

Mechanical impedance

The ratio of force to velocity, where the velocity is a result of that force only. Its reciprocal, mobility is today more favoured.

Mode

A characteristic pattern in a vibrating system. All points reach their maximum x at the same instant.

Natural environments

Conditional occurring in nature; effects are observed whether an equipment is at rest or in operation.

Natural frequency, fn

The frequency of an undamped systemâ€™s free vibration; also the frequency of any of the normal modes of vibration. fn drops when damping is present.

Noise

The total of all interferences in a measurement system, independent of the presence of signal.

Observed mean time between failures (MTBF)

For a stated period in the life of an item, the mean value of the length of time between consecutive failures computed as the ratio of the cumulative observed time to the number of failures under stated conditions.

Observed Mean time to failure (MTTF)

For a stated period in the life of an item, the ratio of the cumulative time for a sample to the total number of failures in the sample during the period under stated conditions.

Octave

The interval between two frequencies differing by exactly 2:1

Oscillation

Variation with time of a quantity such as F, stress, pressure, x, v, a or jerk. Usually implies some regularity (as in sinusoidal or complex vibration).