In measurement science, the word 'standard' is used with two different meanings: first, as a widely adopted specification, technical recommendation or similar document; and, second, as a measurement standard. This note deals with measurement standards, which can be a physical measure, measuring instrument, reference material or measuring system intended to define, realize, conserve or reproduce a unit or one or more values of a quantity to serve as a reference. For example, the unit of the quantity 'mass' is given its physical form by a cylindrical piece of metal of one kilogram, which represents the international standard, and gauge blocks represent certain values of the quantity 'length'.

The hierarchy of measurement standards (see Figure) starts from the international standard at the apex, which is known with the highest precision and goes all the way down to working standards. International measurement standards are standards recognized by an international agreement to serve internationally as the basis for assigning values to other standards of the quantity concerned. The oldest standard in use today is the International Prototype of the Kilogram, kept at the Bureau International des Poids et Mesures (BIPM) in Sèvres. The role of the BIPM is to ensure the international consistency of the highest level measuring standards in each of the signatories of the international treaty known as the Metre Convention.

A national measurement standard is a standard, often a primary standard, recognized by national law to serve in a country as the basis for assigning values to other standards of the quantity concerned. The custodian of national measurement standards in, for example, the USA is the NIST and in the Netherlands it is the NMi.

A primary standard is a standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity. Primary standards are, for example, Josephson devices for the realization of the quantity 'volt', or stabilized lasers used in conjunction with interferometers for the realization of the quantity 'length'. The devices are used as national standards.

Secondary standards are standards whose value is assigned by comparison to a primary standard of the same quantity. Primary standards are usually used to calibrate secondary standards. A working standard is a standard that is used routinely to calibrate or check material measures, measuring instruments or reference materials. A working standard is usually calibrated with reference to a secondary standard, and may be used to ensure that routine measurements are being carried out correctly – a check standard.

A reference standard is a standard generally having the highest metrological quality available at a given location or in a given organization from which the measurements made at that location are derived. Calibration laboratories maintain reference standards for calibrating their working standards.