Question 216534
It is one of the dimensions of a geometric figure, usually the longer one, but not always.
It is mostly used in reference to squares, rectangles, cubes, rectangular solids, cylindrical solids, etc.
dimensions moost often used are:
length
width
height
depth
side
radius
circumference
etc.
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It is also used as a measure, like when you say something is 5 feet long, or something has a length of 3 feet, inches, centimeters, etc.
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The most obvious example would be a rectangle that has a length of 6 feet and a width of 2 feet, for example.
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The length would be two opposite sides of the rectangle that are parallel to each other and perpendicular to the width.  The width would be two sides of the rectangle that are parallel to each other and perpendicular to the length.  The perimeter of the rectangle would be 2 times the length plus 2 times the width.  The area of the rectangle would be the length times the width.
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The length, used in this way, identifies two of the sides of the rectangle as being distinct from the other two sides of the rectangle.