To measure anything accurately - hence to obtain a measurement result or assign a value that is as true, correct or exact as possible - and for this measurement to be repeated anywhere in the world and yield an accurate result, a system of units such as the International System of Units, the SI, is needed. In order to establish such a system, a system of quantities including a set of equations defining the relations between those quantities, must be established. Corresponding units must then be defined as a measure of the quantities. The units must be unique and provide a sound theoretical basis upon which the most accurate and reproducible measurements can be made. Procedures to establish the value and associated uncertainty of the same kind as the unit must be established, called the "realisation of the definition of a unit". Finally, these realisations and the measurement standards (including "weights") established to disseminate the realisations in countries across the world, called national measurement standards (NMS), must be compared to determine their degree of equivalence (or comparability).
If all this is done, a uniform, world-wide measurement system has been established. The process to establish and maintain this system is the main task of the International Bureau of Weights and Measures (BIPM) and the National Metrology Institutes (NMIs). The scientific work associated with the process is called "Scientific Metrology". In the broad context, metrology also include Industrial metrology systems for inspection and quality control and Legal metrology for that part of metrology relating to activities which result from statutory requirements (for example where the protection of consumers are regulated by law).
A core concept in metrology is metrological traceability, defined in the International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM, 3rd edition, JCGM 200:2008) as "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties." The level of traceability establishes the level of comparability of the measurement: whether the result of a measurement can be compared to the previous one, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world.
Traceability is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard (the NMS). These standards can either be primary (Established through a realisation of the unit and the traceability is to the NMI) or secondary, where an artefact calibrated against a primary standard at another NMI is maintained as the NMS (and the traceability is to the NMI with the primary standard). In chemistry, traceability is mostly established through the use of certified reference materials to calibrate the measuring equipment.