The Collaborative International Dictionary of English v.0.48:

nineties \nineties\ n. pl.
   1. The decade from 1890 to 1899; as, the gay nineties.
      [WordNet 1.5]

   2. The decade from 1990 to 1999; as, they spent a ton of
      money to solve the Y2K bug in the nineties.
      [WordNet 1.5]

   3. The time of life between ages 90 and 100.

   Syn: mid-nineties.
        [WordNet 1.5]

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):

nineties
    n 1: the decade from 1890 to 1899 [syn: nineties, 1890s]
    2: the decade from 1990 to 1999 [syn: nineties, 1990s]
    3: the time of life between 90 and 100 [syn: nineties, mid-
       nineties]