A measurement standard or etalon, is a material measure, measuring instrument, reference material or measuring system intended to define, realise, conserve or reproduce a unit or one or more values of a quantity to serve as a reference.
Material measure: Device intended to reproduce or supply, in a permanent manner during its use, one or more known quantity value e.g. a standard weight, a volume measure, a gauge block, or a certified reference material.
Metrological traceability: Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
Uncertainty of measurement: Non-negative parameter, associated with the result of a measurement that characterises the dispersion of the quantity values being attributed to the measurand, based on the information used.
Calibration: Set of operations that establish, under specified conditions, the relationship between the quantity values with measurement uncertainties provided by measurement standards or certified reference materials and corresponding indications with the associated measurement uncertainties of the measurement instrument, measuring system or reference material under test.

Example
The metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The metre is realised at the primary level in terms of the wavelength from an iodine-stabilised helium-neon laser. On sub-levels, material measures like gauge blocks are used, and traceability is ensured by using optical interferometry to determine the length of the gauge blocks with reference to the abovementioned laser light wavelength.