1meter

The meter, symbol m, is the SI base unit of length. It is defined as the distance light travels in vacuum in a time interval of 1/299,792,458 of a second, which fixes the speed of light at exactly 299,792,458 meters per second. This definition ties the unit to a fundamental constant of nature rather than a physical artifact.

Historically, the meter has undergone several redefinitions. In 1799 it was defined as one ten-millionth of the distance from the equator to the North Pole along a meridian through Paris. In 1889 the International Prototype Meter, a platinum–iridium bar, became the standard and was kept at the International Bureau of Weights and Measures (BIPM). In 1960 the meter was defined as 1,650,763.73 wavelengths of the radiation corresponding to the transition between two hyperfine levels of cesium-133 in vacuum. Since 1983 the definition has been based on the fixed speed of light.

As the SI base unit of length, the meter is used worldwide in science, engineering, commerce, and construction. It serves as the reference from which many derived units are formed, such as kilometers, centimeters, and millimeters, and it underpins precise measurements, calibration, and manufacturing standards. In everyday use, practical approximations include 1 meter being about 1.094 yards or 3.2808 feet. The meter remains maintained by international and national metrology institutes, with the BIPM coordinating standards, comparisons, and reproducibility of measurements across laboratories.