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.
Primary measurement standard: A measurement standard established using a primary reference measurement procedure or created as an artefact, chosen by convention. A standard that is designated or widely acknowledged as having the highest metrological qualities and whose measurement results are determined without reference to other standards of the same quantity in the same measurement range.
Secondary standard: Measurement standard established through calibration with respect to a primary measurement standard for a quantity of the same kind.
Working standard: Measurement standard that is routinely used to calibrate or verify measuring instruments or measuring systems.
International measurement standard: Measurement standard recognised by signatories to an international agreement and intended to serve worldwide e.g. the international prototype of the kilogram.
National measurement standard: Measurement standard recognized by national authority to serve in a state or economy as the basis for assigning quantity values to other measurement standards for the kind of quantity concerned.
