In the language of measurement, units measure quantifiable aspects of the world, such as time, distance, velocity, mass, and weight. Units are sometimes also referred to as dimensions although the term is only strictly true of time and space measurements.
A system of fundamental units (or sometimes fundamental dimensions) is such that every other unit can be generated from them. Traditionally, the accepted fundamental units are mass, length, time, and temperature, but in principle, other fundamental units could be used. The kilogram, meter, second, ampere, Kelvin, mole and candela are the fundamental SI units; other units such as the newton, joule, and volt are called derived units as they can be defined in terms of the fundamental units. See SI derived unit
Velocity, for example, is length divided by time, and so can be generated from the above list of fundamental units.
It is an important basic fact of dimensional analysis that the fundamental units can be regarded as the basis of a special kind of vector space, the space of all units. This is a vector space over the field of rational numbers where the vector addition is given by the multiplication of units and the scalar multiplication is exponentiation of units.
Not all physically important values have units: dimensionless numbers occur in many fields of science.