The modern form of the metric system, and the world's most widely used system of measurement. It defines base units for length (meter), mass (kilogram), time (second), and more, with other units derived from these.
A system of units historically used across the British Empire, based on units like the foot, pound, and gallon. The United Kingdom has officially adopted the metric system, though imperial units remain in everyday informal use.
Similar to, but not identical to, the imperial system (notably, US and UK gallons and fluid ounces differ). US customary units remain the primary system used in the United States.