A mill, mil or mille is 1/10th of a US cent or 1/1000th of a US Dollar. In the United States, it is most used in gasoline prices. For example $4.119 per US gallon. $0.009 is equal to nine mils. It is also used when discussing taxes on land and prices of electricity.
I was born in the US in 1950, and raised in the US. When I was a child probably some age less than 10, there were plastic ‘coins’ called ‘Mils’. The red ‘Mils’ were 1/10 of a cent. The green ‘Mils’ were worth 5/10 (1/2) of a cent. I have a green one. My mother with 7 children was always afraid one of us might swallow one and it would not be able to be seen on the then x-ray machines. So we would be in medical danger.