Home > Encyclopedia of Science > Units and Standards

• History
• The metric system
• Words to Know
• Le Système International d'Unités (the SI system)
• Derived units
• Natural units
• Unit conversions between systems

A unit of measurement is some specific quantity that has been chosen as the standard against which other measurements of the same kind are made. For example, the meter is the unit of measurement for length in the metric system. When an object is said to be 4 meters long, that means that the object is four times as long as the unit standard (1 meter).

The term standard refers to the physical object on which the unit of measurement is based. For example, for many years the standard...