Length is generally described as the longest dimension of an object. Length is a measure of one dimension, whereas area is a measure of two dimensions and volume is a measure of three dimensions.
For the imperial system the commonly used units of length are the inch, the foot, the yard, and the mile.
In the metric system the basic unit of length is the metre and is defined in terms of the speed of light. Commondly used units of length in the metric system are metres and kilometers.