The decade of one's life from age 90 through age 99.
(temperature, rates, plural only) The range between 90 and 99.
My ideal bracket of temperature ranges from the sixties up to the nineties.
noun
The decade of the 1890s, 1990s, etc.
1919, Harry Hamilton Johnston, The Gay-Dombeys: A Novel, Macmillan, page 172,
He and his clever staff of minor blackguards exploited to the full every weakness and caries in the London Society of the 'eighties, 'nineties, and 'oughts.
adjective
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.