Definition:Millilitre/Linguistic Note

Linguistic Note on Millilitre
The spelling millilitre is the one adopted by the International Bureau of Weights and Measures. Milliliter is the variant used in standard American English.

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, in order to provide consistency with the British spelling of metre.