The Collaborative International Dictionary of English v.0.48:

Ninety \Nine"ty\, a. [See Nine, and cf. Forty.]
   Nine times ten; eighty-nine and one more; as, ninety men.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ninety \Nine"ty\, n.; pl. Nineties.
   1. The sum of nine times ten; the number greater by a unit
      than eighty-nine; ninety units or objects.
      [1913 Webster]

   2. A symbol representing ninety units, as 90 or xc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ninety
    adj 1: being ten more than eighty [syn: ninety, 90, xc]
    n 1: the cardinal number that is the product of ten and nine
         [syn: ninety, 90, XC]