|SI unit||meter (m)|
|see unit of length|
Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement, the unit of length is a base unit, from which other units are derived.
Types of length include height, which is vertical length or vertical extent, and width or breadth, which are the side-to-side length, measuring across the object at right angles to the height.
Measurement has been important ever since humans settled from nomadic lifestyles and started using building materials, occupying land and trading with neighbours.
As trade between different places increased, the need for standard units of length increased. And later, as society has become more technologically oriented, much higher accuracy of measurement is required in an increasingly diverse set of fields, from micro-electronics to interplanetary ranging.
Under Albert Einstein's special relativity, length can no longer be thought of as being constant in all reference frames. Thus a ruler that is one meter long in one frame of reference will not be one meter long in a reference frame that is moving relative to the first frame. This means the length of an object varies depending on the observer.
In the physical sciences and engineering, when one speaks of units of length, the word length is synonymous with distance. There are several units that are used to measure length. Historically, units of length may have been derived from the lengths of human body parts, the distance traveled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some common object.
In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light. The centimetre and the kilometre, derived from the metre, are also commonly used units. In U.S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.