Metre
SI unit of length
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 1/299792458 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 10000 km 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 1/299792458 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 ΔνCs.
Etymology and spelling
The etymological roots of metre can be traced to the Greek verb μετρέω (metreo) ((I) measure, count or compare) and noun μέτρον (metron) (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 (metior, mensura), French (mètre, mesure), English (meter for measuring instruments, but metre or meter in poetry) and other languages.
The public source identifies “Metre” as sI unit of length. This brief keeps that definition visible, then builds a research path around Metre, unit and length.
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The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Aug 29, 2026. The linked authority identifier is Q11573. The Library of Congress control number is sh85084371. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank. The first chronological checks are 2019, 1791, 1799 and 1889.
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This entry incorporates text from “Metre” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.