A per mil, per mille, or per mill (permil or permille) (Latin, literally meaning 'for (every) thousand') is a tenth of a percent or one part per thousand. It is written with U+2030‰per mille sign (HTML: ‰‰) (Windows: ALT+0137), which looks like a percent sign (%) with an extra zero at the end. It can be seen as a stylized form of the three zeros in the denominator, although it originates from an alteration of the percent sign.
The term occurs so rarely in English that major dictionaries do not agree on the spelling or pronunciation (even within one dialect of English) and often do not list the other variants or do not even have an entry (for example Longman, and Macmillan). The term is very common in most other languages, for example in expressing blood alcohol content, usually expressed in percentage terms in English-speaking countries.
A per mil is defined as:
1‰ = 10−3= 1⁄1000 = 0.001 = 0.1%
1% = 10‰
A per mil should not be confused with ppm (commonly pronounced "parts per million"). A measurement in ppm means parts per million, and has as its denominator 1,000,000 and not 1,000.