Plural of ninety
The decade of the 1890s, 1990s, etc.
The decade of one's life from age 90 through age 99.
(temperature, rates) The range between 90 and 99.
From or evoking the 91st through 100th years of a century (chiefly the 1990s).
I found myself attracted to his mullet even though it struck me as needlessly nineties.