Ninety is the number that comes after eighty-nine and before ninety-one. When you write it as numerals, it looks like this: 90.

Ninety represents nine groups of ten. If you had ninety baseball cards, you could organize them into nine stacks of ten cards each. It's the product of multiplying 9 × 10, which makes it easy to work with in math.

The number ninety shows up in many familiar contexts. A right angle, like the corner of a square or the angle your body makes when you sit up straight, measures ninety degrees. Many school tests are graded out of 100 points, so scoring ninety points means you got 90% correct, usually an A. In sports, basketball games in the NBA last forty-eight minutes, but high-scoring teams often score ninety or more points.

When people talk about “the nineties,” they usually mean the years from 1990 to 1999. Your parents or older siblings might have been born in the nineties. Some people also use “the nineties” to describe someone's age: a person in their nineties is between 90 and 99 years old.