The square root of 81 is 9. This means that when 9 is multiplied by itself (9 x 9), it equals 81. The concept of square roots is foundational in mathematics, where the square root of a number is a value that, when squared, gives the original number. In various mathematical applications, understanding square roots can help solve equations and understand geometric properties. For instance, in geometry, the square root concept is often used when calculating the length of sides in right triangles using the Pythagorean theorem.