Length is the measurement of something from end to end or along its longest side. In geometric measurements, length is the most extended dimension of an object.
In the International System of Quantities, length is any quantity with dimension distance. In other contexts, length is a measured dimension of an object. In mechanics, the three basic quantities are length (L), mass (M), and time (T). All other quantities in mechanics can be expressed in terms of these three.

Length: Definition 1

In A.D. 1120 the king of England decreed that the standard of length in his country would be named the yard and would be precisely equal to the distance from the tip of his nose to the end of his outstretched arm.
Fig. 1: Defining yard as a unit of length.

Length: Definition 2

Similarly, the original standard for the foot adopted by the French was the length of the royal foot of King Louis XIV.
Fig. 2: Defining foot as a unit of length.

Length: Definition 3

King Louis XIV standard prevailed until 1799, when the legal standard of length in France became the meter, defined as one ten-millionth the distance from the equator to the North Pole along one particular longitudinal line that passes through Paris. Many other systems for measuring length have been developed over the years, but the advantages of the French system have caused it to prevail in almost all countries and in scientific circles everywhere.
Fig. 3: Meter was defined as a unit of length by the first time in the history.

Length: Definition 4

As recently as 1960, the length of the meter was defined as the distance between two lines on a specific platinum iridium bar stored under controlled conditions in France. This standard was abandoned for several reasons, a principal one being that the limited accuracy with which the separation between the lines on the bar can be determined does not meet the current requirements of science and technology.
Fig. 4: Platinum iridium bar stored under controlled conditions in Paris France.

Length: Definition 5

In the 1960s and 1970s, the meter was defined as 1 650 763.73 wavelengths of orange-red light emitted from a krypton-86 lamp.
Fig. 5: Light emitting from a krypton-86 lamp.

Length: Definition 6

However, in October 1983, the meter (m) was redefined as the distance traveled by light in vacuum during a time of 1/299 792 458 second. In effect, this latest definition establishes that the speed of light in vacuum is precisely 299 792 458 m per second.
Fig. 6:The speed of light in vacuum.

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