—n. 1. a cardinal number, ten times nine.
2. a symbol for this number, as 90 or XC.
3. a set of this many persons or things.
4. nineties,the numbers, years, degrees, or the like, from 90 through 99, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature: His grandmother is in her nineties.