The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of a second, where the second is defined by a hyperfine transition frequency of caesium.
The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris, setting as that quarter of the Earth's polar circumference.
In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in vacuum in of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency . This series of amendments did not alter the size of the metre significantly â modern measurements of the Earth's polar circumference give a figure of .
Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States and the Philippines which use meter.
Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English. The suffix "-meter" has the same Greek origin as the unit of length.
The etymological roots of metre can be traced to the Greek verb () ((I) measure, count or compare) and noun () (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (), French (), English and other languages. The Greek word is derived from the Proto-Indo-European root ' 'to measure'. In English, the use of the word metre (for the French unit ) began at least as early as 1797.
SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, astronomical units (), light-years (), or parsecs (), rather than in Mm or larger multiples. "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.
The terms micron and millimicron have been used instead of micrometre (üm) and nanometre (nm), respectively, but this practice is discouraged.
Within this table, "inch" and "yard" mean "international inch" and "international yard" respectively, though approximate conversions in the left column hold for both international and survey units.
One metre is exactly equivalent to inches and to yards.
A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet inches. This gives an overestimate of 0.125 mm.
The ancient Egyptian cubit was about 0.5 m (surviving rods are 523âÂÂ529 mm). Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.