meter Symbol m. SI unit of length. The meter is the length of path traveled by light in vacuum during a time interval 1/299 792 458 of a second.

Note that the above definition has been adapted in 1983. Prior to 1960, the meter was defined as the distance between two scratches on a platinum iridium bar stored under controlled condition at the international bureau of weights and measures in Sevres, France. The standard meter cannot be reproduced easily. Secondly the width of the scratches is a limiting factor. The standard meter was measured precisely in terms of number of wavelengths of the orange light emitted by krypton-86. The meter was defined as 165 076 3.73 wavelengths of this light. But the precision of the wavelength measurement itself was a limiting factor. Hence the present definition has been adapted. The speed of light has therefore become a primary standard and taken exactly equal to 299 792 458 m/s.