Definition:Metre/Linguistic Note

Linguistic Note on Metre
The word metre originated with the Italian scientist 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 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.