a cardinal number, ten times nine.

a symbol for this number, as 90 or XC.

a set of this many persons or things.

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.

amounting to 90 in number.