Unit of length
A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units. Metric system. SI. The base unit in the International System of Units (SI) is the metre, defined as "the length of the path  travelled by light in vacuum during a time interval of seconds." It is approximately equal to . Other units are derived from the metre by adding prefixes from the table below: For example, a kilometre is . A slang term for the kilometre in the US and UK militaries is "klick". Non-SI. In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre. Other non-SI units are derived from decimal multiples of the metre. Imperial/US. The basic unit of length in the imperial and U.S. customary systems is the yard , defined as exactly by international treaty in 1959. Common imperial units and U.S. customary units of length include: Marine. In addition, the following are used by sailors: Aviation. Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance. Surveying. Surveyors in the United States continue to use: Science. Astronomy. Astronomical measure uses: Physics. In atomic physics, sub-atomic physics, and cosmology, the preferred unit of length is often related to a chosen fundamental physical constant, or combination thereof. This is often a characteristic radius or wavelength of a particle. Some common natural units of  length are included in this table: Archaic. Archaic units of distance include: Informal. In everyday conversation, and in informal literature, it is common to see lengths measured in units of objects of which everyone knows the approximate width. Common examples are: Other. Horse racing and other equestrian activities keep alive: