Units and standards for expressing the amount of some quantity; such as, length, capacity, or weight.

The science of measurement standards and methods is known as metrology.

Today the chief systems are the English units of measurement and the metric system

The United States is one of the few countries still using the English system; all other major nations have either converted to the metric system or committed themselves to conversion.

The English system is much older and is said to be less practical than the metric system, and in the United States there has been considerable discussion in favor of adopting the metric system as the principal system; however, attempts to legislate such a change in the U.S. Congress have failed.

The basic units of the English system, the yard of length and the pound of mass, are now defined in terms of the metric standards, the meter of length and the kilogram of mass.