<P> A base unit (also referred to as a fundamental unit) is a unit adopted for measurement of a base quantity . A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others . </P> <P> In the language of measurement, quantities are quantifiable aspects of the world, such as time, distance, velocity, mass, temperature, energy, and weight, and units are used to describe their magnitude or quantity . Many of these quantities are related to each other by various physical laws, and as a result the units of a quantities can be generally be expressed as a product of powers of other units; for example, momentum is mass multiplied by velocity, while velocity is measured in distance divided by time . These relationships are discussed in dimensional analysis . Those that can be expressed in this fashion in terms of the base units are called derived units (fr). </P> <P> In the International System of Units, there are seven base units: kilogram, metre, candela, second, ampere, kelvin, and mole . </P> <P> There are other relationships between physical quantities that can be expressed by means of fundamental constants, and to some extent it is an arbitrary decision whether to retain the fundamental constant as a quantity with dimensions or simply to define it as unity or a fixed dimensionless number, and reduce the number of explicit fundamental constants by one . </P>

What is a base unit and why do we need one