millesimal

noun

mil·​les·​i·​mal mə-ˈle-sə-məl 

: the quotient of a unit divided by 1000 : one of 1000 equal parts

millesimal adjective

millesimally

mə-ˈle-sə-mə-lē 

adverb

Word History

Etymology

Latin millesimus, adjective, thousandth, from mille

First Known Use

1719, in the meaning defined above