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)