A system of measurement is a set of units which can be used to specify anything which can be measured

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 and were historically important, regulated and defined because of trade

Trade

Trade is the transfer of ownership of goods and services from one person or entity to another. Trade is sometimes loosely called commerce or financial transaction or barter. A network that allows trade is called a market. The original form of trade was barter, the direct exchange of goods and...

 and internal commerce

Commerce

While business refers to the value-creating activities of an organization for profit, commerce means the whole system of an economy that constitutes an environment for business. The system includes legal, economic, political, social, cultural, and technological systems that are in operation in any...

. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them, whereas in the early and most historic eras, the units were given by fiat (See Statutory law

Statutory law

Statutory law or statute law is written law set down by a legislature or by a legislator .Statutes may originate with national, state legislatures or local municipalities...

) by the ruling entities and were not necessarily well inter-related or self-consistent.

Although we might suggest that the Egyptians had discovered the art of measurement, it is only with the Greeks that the science of measurement begins to appear.