The $n^{\text{th}}$ root of a number $a$ expresses another number that, when multiplied by itself $n$ times, will result in $a.$ Aside from the radical symbol, the notation is made up of the radicand $a$ and the index $n.$

The resulting number is commonly called a radical. For example, the radical expression $\sqrt[4]{16}$ is the fourth root of $16.$ $\sqrt[4]{16}$ simplifies to $2$ because $2$ multiplied by itself $4$ times equals $16.$ $$\begin{array}{c}\sqrt[4]{16}=\sqrt[4]{2^4}=2\end{array}$$ The general expression $\sqrt[n]{a}$ represents a number which equals $a$ when multiplied by itself $n$ times.

For any real number $a$ and natural number $n,$ the expression $a^{\frac{1}{n}}$ is defined as the $n^{\text{th}}$ root of $a.$ Note that a root with an even index is defined only for non-negative numbers. Therefore, if $n$ is even, then $a$ must be non-negative.

Just as with exponents, the most common roots have special names: square roots and cube roots have an index of $2$ and $3,$ respectively.