In general English usage, length is but one particular instance of distance – an object's length is how long the object is – but in the physical sciences and engineering, the word length is in some contexts used synonymously with "distance". Height is the term for vertical length, width is a lateral distance; an object's width is less than its length. No one speaks of "the length from here to Alpha centauri", but rather of "the distance from here to Alpha centauri," but when one speaks of distance more abstractly, one says "A mile, or a kilometer, is a unit of length" or "...of distance", and the two statements are synonymous. Likewise, a mountain might be a mile in height. Length is the metric of one dimension of space. The metric of space itself is volume, or (length)3. Length is commonly considered to be one of the fundamental units, meaning that it cannot be defined in terms of other dimensions. However, a set of units can be constructed where length is dimensionless – see Planck units.

Length is not an intrinsic property of anything, however, in that two observers can measure the same "thing" (i.e. distance between events, length of a board) and come up with a different answer. This strange property of space is explained by Albert Einstein's special theory of relativity.

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