Ninety is a cardinal number that represents the quantity of 90. This number can also be written as ninety, which is the English word that describes it. Ninety is often used to refer to an age, such as "my grandmother is ninety years old." In addition, it is used in measurements, such as "the temperature today is ninety degrees Fahrenheit." The word ninety is also part of many other English words, such as ninety-five, ninety-six, and ninety-nine. Overall, the use of the word ninety is an important part of the English language and is used in various contexts to describe a quantity, measurement or age.