In math, inverse is explained as the opposite in effect. An inverse function is one that reverses another function. If an inverse does exist for a function, it is referred to as an invertible function.
The inverse of the operation of addition is subtraction. An example of this would be the following reversal: the inverse of 20 + 9 = 29 is 29 - 9 = 20.
The operation of multiplication has an inverse operation of division. Multiplication can essentially be reversed by division. An example of this is 5 multiplied by 9 equals 45. The inverse of this would be 45 divided by 9 equals 5.