The classical definition of measurement is 'the determination or estimation of ratios of quantities'. Quantity and measurement can bemutually defined:

- Quantitative attributes are those possible to measure, at least in principle.'
- Another useful definition is ' a method of determining quantity, capacity or dimension.
- Several systems of measurement exist, each one comprising units whose amounts have been arbitrarily set and agreed upon by specific groups.'

- In representational theory, measurement is defined as the 'correlation of numbers with entities that are not numbers.
- In information theory, measurement is defined as a 'set of observations that reduce uncertainty where the result is expressed as a quantity.'
- Quantum mechanics defines measurement as 'an action that determines the location of an object , its momentum, its polarity etc.'
- A system of measurements is 'a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce.'
- The Imperial System was the 'traditional system of weights and measures used officially in Great Britain from 1824 until the adoption of the metric system beginning in 1965.' Imperial quantities included inches, feet and yards are used for length, ounces and pounds for mass, pound-force for force.
- The Metric System was devised by French scientists in the late 18th century. Metres (m) are used for length, kilograms (kg) for mass, seconds (s) for time, ampere (A) for electric current, kelvins (K) for thermodynamic temperature, mole (mol) for amount of substance and candela (cd) for luminous intensity.