Fundamental units:

There are certain units of measurement which are considered fundamental in which they cannot be described as a function of other units. Distance, mass and time are the fundamental units set of fundamental units are a set of units for physical quantities from which every other unit can be generated. In measurement, quantities like time, velocity, mass, distance, momentum, energy, and weight, and units are used to describe their measure. A number of these quantities are related to each other by various physical laws, and as a result the units of some of the quantities can be expressed as products (or ratios) of powers of other units. Those that cannot be so expressed can be regarded as "fundamental" in this sense.

For instance, distance and time are related to each other by speed of light, c, which is a fundamental constant. It is possible to use this relationship to remove either the fundamental unit of time or that of distance. Similar considerations apply to Planck's constant, h, which relates energy to frequency. In SI system there are 7 fundamental units: meter, candela, kilogram, ampere, second, Kelvin, and mole. In theory a system of fundamental quantities would be such that every other physical quantity can be obtained from them.

One could eliminate any two of the kilogram, meter, and second by setting c and h to unity or to a fixed dimensionless number. One could then remove the ampere either by setting the permittivity of free space to a fixed dimensionless number or by setting the electronic charge to such a number. One could similarly remove the mole as a fundamental unit by reference to Avogadro's number. One could remove the Kelvin as it can be argued that temperature simply expresses the energy per particle per degree of freedom which can be expressed in terms of energy.