Metre
A metre (US spelling, meter) is the basic unit of length in the SI measurement system. The abbreviation for the metre is m. The first meaning (in the French Revolution), it was 1/10,000,000th (one tenmillionth) of the distance between the Earth's equator and the North Pole along the Paris meridian. The metre is now officially defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
In the imperial system of measurement, one metre is equal to about 39.37 inches, 3.28 feet, or about 1.09 yards.
Units based on the metre
 0.000 000 000 000 000 000 000 001 Ym (yottametre) = 1 m
 0.000 000 000 000 000 000 001 Zm (zettametre) = 1 m
 0.000 000 000 000 000 001 Em (exametre) = 1 m
 0.000 000 000 000 001 Pm (petametre) = 1 m
 0.000 000 000 001 Tm (terametre) = 1 m
 0.000 000 001 Gm (gigametre) = 1 m
 0.000 001 Mm (megametre) = 1 m
 0.001 km (kilometre) = 1 m
 0.01 hm (hectometre) = 1 m
 0.1 dam(decametre) = 1 m
 1 m (metre)
 10 dm (decimetres) = 1 m
 100 cm (centimetres) = 1 m
 1000 mm (millimetres) = 1 m
 1 000 000 μm (micron or micrometres) = 1 m
 1 000 000 000 nm (nanometres) = 1 m
 1 000 000 000 000 pm (picometres) = 1 m
 1 000 000 000 000 000 fm (femtometres) = 1 m
 1 000 000 000 000 000 000 am (attometres) = 1 m
 1 000 000 000 000 000 000 000 zm (zeptometres) = 1 m
 1 000 000 000 000 000 000 000 000 ym (yoctometres) = 1 m
*Note: units in bold are the most commonly used.
Images

Closeup of National Prototype Metre Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) and given to the United States, which served as the standard for defining all units of length in the US from 1893 to 1960

Belfry, Dunkirk—the northern end of the meridian arc