1. The problem is to simplify the square root of 81. 2. The square root function, denoted as $\sqrt{x}$, gives the number which, when multiplied by itself, equals $x$. 3. We know that $81 = 9 \times 9$. 4. Therefore, $\sqrt{81} = 9$ because $9 \times 9 = 81$. 5. So, the simplified form of $\sqrt{81}$ is $9$.