Small number to the lower right of the letter it characterizes and that is generally used to number it.

- It is important not to confuse the terms “index” and “exponent”, since “\(x_{2}\)” is read as “\(x\) index 2″ and “\({x}^{2}\)” is read as “
*x*exponent 2″. - There is also the index of a radical; in the expression \(\sqrt[3]{27}\), the number 3 is the index of the radical. When the radical does not have an index, as in \(\sqrt{27}\), the index of the radical is understood to be 2.

### Notation

In the expression \(x_{1}\), the number 1 is written as an index of the letter \(x\) and is read “\(x\) index 1″.