The system of
imperial units or the
imperial system (also known as
British Imperial) is the
system of units first defined in the British
Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the
British Empire. By the late 20th century, most nations of the former empire had
officially adopted the
metric system as their main system of measurement; however some imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as
English units, as did the separate system of
United States customary units.