The mole and the International System of Units (SI)

Metrology is the "science of measurement and its applications" (VIM §2.2). In fact, historically, metrology had more of a political purpose: the aim was to establish a single system of measurement units that would regulate a country's technical and economic activities. It was in China, during the 2 ^e century BC, that the first single system of measurement and currency was created. Today, we speak of the International System of Units, or SI, because the vast majority of countries have adopted it. There are, however, a few obsolete persisters, such as the gallon or the mile in the USA, or the barrel for oil tankers and the degree Celsius in chemistry.

If you work in an accredited laboratory, regularly audited by CoFRAC, ISO 17025 requires you to "ensure the traceability of calibrations and measurements carried out by the laboratory to the International System of Units (SI)". So it's worth taking a look at how the SI works.