Definition of per mill in English:

per mill

noun

(also per mil)
  • One part in every thousand.

    ‘Isotopic concentrations were expressed as delta values in per mill.’
    • ‘He says the loss of sales per day for the industry is around 2 per day per mill.’
    • ‘According to the bylaw, the levy will be one per mill of the value of the animal.’

Origin

Late 17th century Latin.