Definition of Unit and its Types

Measurement is fundamentally an act of comparison. To say a rope is '10 metres long' is to say it is 10 times as long as an object whose length has been defined as 1 metre -- and that reference length, 1 metre, is the unit of the quantity 'length'.

An arbitrarily chosen standard of measurement of a quantity, accepted internationally, is called the unit of that quantity.

Units split into two types, mirroring the split among physical quantities:

• Fundamental (base) units -- the units in which the fundamental quantities are measured (metre, kilogram, second, ...). …