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 ]