|Definition:|| real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the second quantity to the first one as a number
NOTE 1 – Units of measurement are designated by conventionally assigned names and symbols.
NOTE 2 – Units of quantities of the same dimension may be designated by the same name and symbol even when the quantities are not of the same kind. For example joule per kelvin and J/K are respectively the name and symbol of both a unit of heat capacity and a unit of entropy, which are generally not considered to be quantities of the same kind. Another example is the unit ohm (Ω) for both electric resistance and electric impedance. However, in some cases special unit names are restricted to be used with quantities of specific kind only. For example, the unit second to the power minus one (1/s) is called hertz (Hz) when used for frequencies and becquerel (Bq) when used for activities of radionuclides. Another example is joule (J), used for energy, but never for moment of force, the unit of which is newton metre (N•m).
NOTE 3 – Units of quantities of dimension one are numbers. In some cases, these units are given special names, e.g. radian (rad), steradian (sr), and decibel (dB), or are expressed by quotients such as millimole per mole (mmol/mol) equal to 10–3, and microgram per kilogram (µg/kg) equal to 10–9.
NOTE 4 – For a given quantity, the short term “unit” is often combined with the quantity name, such as "unit of mass".