Measuring a quantity means comparing it to a quantity of the same type taken as a reference. This latter quantity constitutes a unit of measurement; the set of legal units of measurement constitute a system known as the Système International (SI).

In metrology, the science of measurement, the term measure or measurement has a number of meanings which it is worth noting here; it may refer to:

  • a value: the measurement of a distance is 100 metres
  • a result: measurement to within 1%
  • an action: making a measurement of electric power
  • an instrument: a capacity measure

To avoid ambiguity, it is preferable to refer to the value of a quantity, the result of a measurement, measuring and measuring equipment. However, where there is no possibility of confusion, a measurement refers to the result of measuring.