Definition 3.3 A unit of measurement is the “standard measure of the physical quantity of interest”, i.e., the number of times the unit occurs in any given amount of the same quantity is the measurement itself (JCGM 100:2008 2008). Without the unit, the measurement number has no physical meaning.
‘One is the name of a certain known quantity of the same kind as the quantity to be expressed, which is to be taken as a standard of reference. (…)’ The first, he said, is called the Unit; (…). Maxwell thought of the unit, ‘the magnitude of which is agreed by men’, as invariant. He used coherent units, so that all his equations were between numerical values.
The VIM does not define unit in an abstract sense; it defines unit of measurement: a ‘particular [measurable] quantity, defined and adopted by convention, with which other [measurable] quantities of the same kind are compared in order to express their magnitudes relative to that quantity.’
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