When the square root of a number is squared, the resulting number is the original number. Since 4²=16, the square root of 16 is 4. The square root function is the inverse of the squaring function just as subtraction is the inverse of addition. To undo squaring, we take the square root.

In general terms, if a is a positive real number, then the square root of a is a number that, when multiplied by itself, gives a. The square root could be positive or negative because multiplying two negative numbers gives a positive number. The principal square root is the non-negative number that when multiplied by itself equals a. The square root obtained using a calculator is the principal square root.

The principal square root of a is written as √​a​​​. The symbol is called a radical, the term under the symbol is called the radicand, and the entire expression is called a radical expression.