# Definition:Metre

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## Metre

The metre (US English: meter) is the SI base unit of length.

It is defined as the distance travelled by light in vacuum in $\dfrac 1 {299 \ 792 \ 458}$ of a second.

The symbol for the metre is $\mathrm m$.

## Square Metre

The square metre is the SI unit of area.

The symbol for the square metre is $\mathrm m^2$.

## Cubic Metre

The cubic metre is the SI unit of volume.

The symbol for the cubic metre is $\mathrm m^3$.

## Historical Note

The metre was initially defined as the length of a pendulum whose period is $1$ second.

It was soon established that as Acceleration Due to Gravity varies considerably according to location, this was not a sustainable definition to maintain a standard.

Hence it was changed so as to be defined as $10^{-7}$ the distance from the Earth's equator, through Paris to the North Pole (at sea level).

This definition was changed again in $1983$ to be defined as the distance travelled by light in vacuum in $\dfrac 1 {299 \ 792 \ 458}$ of a second.

## Linguistic Note

The word metre originated with the Italian scientist Tito Livio Burattini who pioneered the concept of a universal set of fundamental units. He used the term metro cattolico from the Greek μέτρον καθολικόν (métron katholikón), that is universal measure. This word gave rise to the French word mètre which was introduced into the English language in $1797$.

The spelling metre is the one adopted by the International Bureau of Weights and Measures. Meter is the variant used in standard American English, but can be confused for the word for a general device used to measure something, in particular the standard household electricity meter, water meter and so on.

While $\mathsf{Pr} \infty \mathsf{fWiki}$ attempts in general to standardise on American English, the name of this unit is one place where a deliberate decision has been made to use the international spelling.