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The length of a line segment is the distance between its endpoints. Length may be measured in meters, yards, abstract units, etc. For example, in the following diagram
$AB$ is one unit long, $BC$ is two units long, $CD$ is a fifth of a unit, $DE$ is four fifths of a unit, $AC$ is three units, etc.
The concept of length as defined above is a special case of a general concept called measure.
In two-dimensional space, length usually goes along the $x$ axis while height goes along the $y$ axis. The same holds in three-dimensional space.
For triangles, pentagons, and higher $n$ -gons, it is customary to refer to the length of any of its sides as the length of the polygon. The length of a circle's side is called its circumference.
In set theory, length refers to the number of elements a set (or one-dimensional array) has. This is also known as cardinality. For example, $\{2, 5, 11, 23, 47\}$ has length $5$ .
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