Editing Physical quantity (section)

== Dimensions, units, and kind {{anchor|Units and dimensions}} ==

=== Dimensions ===
{{Main|Dimension (physics)}}
The notion of ''dimension'' of a physical quantity was introduced by [[Joseph Fourier]] in 1822.<ref>Fourier, Joseph. ''[[Théorie analytique de la chaleur]]'', Firmin Didot, Paris, 1822. (In this book, Fourier introduces the concept of ''physical dimensions'' for the physical quantities.)</ref> By convention, physical quantities are organized in a dimensional system built upon base quantities, each of which is regarded as having its own dimension.

=== Unit ===
{{main|Units of measurement}} There is often a choice of unit, though [[SI]] [[Units of measurement|units]] are usually used in scientific contexts due to their ease of use, international familiarity and prescription. For example, a quantity of mass might be represented by the symbol ''m'', and could be expressed in the units [[kilogram]]s (kg), [[Pound (mass)|pound]]s (lb), or [[Dalton (unit)|dalton]]s (Da).

=== Kind ===
[[Dimensional homogeneity]] is not necessarily sufficient for quantities to be comparable;<ref name="ISO 80000-1"/>
for example, both [[kinematic viscosity]] and [[thermal diffusivity]] have dimension of square length per time (in units of [[metre squared per second|m<sup>2</sup>/s]]).
Quantities of the same '''''kind''''' share extra commonalities beyond their dimension and units allowing their comparison;
for example, not all [[dimensionless quantities]] are of the same kind.<ref name="ISO 80000-1"/>