Metrology has many terms that are often confusing or inaccessible for newcomers.  This glossary will help explain some of the terms that are used on this website, in the context of our products and in the industry in general.

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The science of measurement.  Often confused with meteorology in polite conversation.

Whilst metrology is a catch all term that refers to measurement in general, in particular the terms below are generally used in the context of industrial dimensional metrology - that is, the measurement of physical dimensions (i.e. the size of things) within an industrial environment (such as manufacturing).

Arc Second

An arc second is a unit of measurement that describes the magnitude of an angle.  It is typically used for small angles that can't be practically described using degrees as a unit of measurement and is 1/60th of an arc minute.  An arc minute is 1/60th of a degree.  Therefore an arc second is 1/3600th of a degree which is equivalent to 0.00028 degrees.

Axial Runout

Axial Runout is the error between an item and the central axis caused as a result of the rotating component not being parallel.  In other words, an angular error in the component will contribute to greater axial runout.


Calibration is the process by which a measurement device is checked for accuracy against a known standard.  This is typically against a measurement artefact that has been traceably calibrated using another device.


Concentricity refers to two circular objects sharing the same center of axis.  Ensuring concentricity is beneficial in large rotating parts such as turbines to increase efficiency and reliability whilst reducing vibration.


A datum is a reference point against which measurements are made.  Usually it is the starting point of all measurement points taken relating to that feature.

Gage R&R

A gage R&R (reliability and repeatability) is a test of performance of a measurement system.  By conducting a series of controlled tests, it indicates measurement variance caused by differing factors in the process including Operators, Equipment, Parts and Process used.

A Gage R&R is conducted to ensure that a piece of measurement equipment is suitable for the process it is intended to be used for.


GD&T stands for Gage Dimensioning and Tolerancing.  It is the method by which dimensions, including nominal values and tolerances are annotated in engineering drawings.


A micon (also known as micrometer) is one thousandth of a milimeter (mm).  This is equivalent to approximately 0.4 thousandths of an inch.

Radial Runout

Radial runout is error caused between a rotating component and the rotating axis caused by the component being positioned off centre from the axis of measurement.


Repeatability refers to the closeness to which a measurement instrument will read the same value for a defined measurement when multiple measurements are collected.  It doesn't take into account measurement of different components or features, nor different operators.


Reproducibility is a metric of the consistency of a measrement of the same part or feature under controlled conditions by multiple operators.


Runout is a form of error that exists in all rotating components whereby there is a variation in circularity from the main axis of the point of rotation.  This is defined as either axial runout or radial runout depending upon the point of measurement.


Swash is an interchangeable term that is synonymous with runout (see above).


Traceability is a key principle of metrology.  All measurement devices and artefacts should be traceable to another that is known and traceably calibraated.  In turn, so should this device be.  By maintaining a trail of traceability (which ultimately follows to the source at a National Measurement Institute), measurement performance is maintained at the expected level.


No measurement is an absolute value - variations will always exist and be contributed to through various factors.  Uncertainty is a definition of how much contribution this equates to in a measurement process.