nine n. in a percentage between 99 and 100, a place on either side of the decimal point given a value of nine and counted in serial to indicate a number very near 100 percent, such as in measurements of reliability, accuracy, purity, etc. Editorial Note: Common forms are five nines, meaning 99.999%, and six nines, meaning 99.9999%. Also used attributively: the company offers five-nines reliability guarantees. (source: Double-Tongued Dictionary)
Further reading
Mary Beth in Greenville, South Carolina, wonders: Why do we say four-oh-nine for the number 409 instead of four-zero-nine or four-aught-nine? What are the rules for saying either zero or oh or aught or ought to indicate that arithmetical symbol...
Astronauts returning from space say they experience what’s called the overview effect, a new understanding of the fragility of our planet and our need to reflect on what humans all share as a species. A book about the end of the universe...