length

Length is a physical quantity that describes the extent of an object or the distance between two points along a straight line. In geometry it is the measure of how long something is, while in physics it serves as a fundamental descriptor of size, position, and motion.

SI base unit: the meter. Since 1983 the meter is defined as the distance that light travels in vacuum in 1/299,792,458 of a second. Earlier definitions tied the meter to physical artifacts or to the size of the Earth, such as one ten-millionth of the distance from the equator to the North Pole along a meridian.

Common units include the kilometer, meter, centimeter, millimeter, micrometer, and nanometer. In everyday use, customary units such as inch, foot, yard, mile, and nautical mile are also employed. Prefixes such as kilo-, deci-, and milli- indicate powers of ten.

Measuring length relies on instruments ranging from simple rulers and tapes to vernier calipers and micrometers for small scales, to coordinate measuring machines for complex parts. Precision length standards and gauge blocks provide traceability to the meter. For high-precision work, optical methods such as laser interferometry are used to compare lengths with sub-micrometer accuracy.

Length is essential in science, engineering, surveying, manufacturing, and everyday activities. In relativistic physics, measured length can depend on the observer's frame of reference, a phenomenon known as length contraction at high speeds.