noun
the '90s; the nineties
Refers to the decade from 1990 to 1999. Commonly used in historical, cultural, or nostalgic contexts.
９０年代の音楽が好きです。
I like music from the '90s.
彼は９０年代に生まれました。
He was born in the nineties.
Refers to the 1980s; same pattern with different decade number.
Refers to the 2000s; note the reading change for the year 2000.
Compound of ９０ (ninety) and 年代 (decade, era). The reading きゅうじゅう is the standard on'yomi for 90.