Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end. In the physical sciences and engineering, the word "length" is typically used synonymously with "distance", with symbol l or L.
Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed). In most systems of measurements, length is a fundamental unit, from which other units are derived.
Units of length
There are several units that are used to measure length. Units of length may be based on lengths of human body parts, the distance travelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some fixed 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 the English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.
Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the astronomical unit, the light-year, and the parsec.
Units used to denote microscopically small distances, as in chemistry, include the micron and the ångström.
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