1. (数字)90
    Ninety is the number 90.

    e.g. It was decided she had to stay another ninety days.
           已经决定了，她不得不再呆90天。

2. 九十多;九十几
    When you talk about the nineties, you are referring to numbers between 90 and 99. For example, if you are in your nineties, you are aged between 90 and 99. If the temperature is in the nineties, the temperature is between 90 and 99 degrees.

    e.g. By this time she was in her nineties and needed help more and more frequently.
           到这个时候她已经90多岁了，越来越频繁地需要帮助。

3. 九十年代(指1990年至1999年)
    The nineties is the decade between 1990 and 1999.

    e.g. These trends only got worse as we moved into the nineties.
           我们步入20世纪90年代时，这些潮流变得更糟了。