Metrology is the science of measurements with which includes all theoretical and practical aspects of measurement.
Concerns the establishment of quantity systems, unit systems, units of measurement, the development of new measurement methods, realization of measurement standards, and the transfer of traceability from these standards to users in society.
Legal metrology "concerns activities which result from statutory requirements and concern measurement, units of measurement, measuring instruments and methods of measurement and which are performed by competent bodies. Such statutory requirements might arise from, amongst others, the needs for protection of health, public safety, the environment, enabling taxation, protection of consumers and fair trade.
Applied, technical or industrial metrology concerns the application of measurement science to manufacturing and other processes and their use in society, ensuring the suitability of measurement instruments, their calibration and quality control of measurements. Although the emphasis in this area of metrology is on the measurements themselves, traceability of the calibration of the measurement devices is necessary to ensure confidence in the measurements.
Comparison of measuring quantity against a standard value of higher accuracy to detect, correlate, adjust, rectify, and document the accuracy of the quantity being compared.