s, 0, 1, 9 are letters that make '1990s'
99, 19, 90 are words within '1990s'
1 possible definitions for '1990s'
Under this definition, 1990s is a noun and has the following properties:
nineties has the same meaning as 1990s
'1990s' can be a type of decade, decennary, decennium.