https://en.m.wikipedia.org/wiki/Inverse_function
The inverse of multiplying by 3 is dividing by 3, and vice-versa.
The inverse of squaring is squart root, and vice-versa.
The inverse of a number or other element (rather than an operation / function) means whatever number would form the inverse if you use it with whatever binary operation you're talking about. So the additive inverse of 3 is –3, the inverse of "rotate clockwise 90 degrees" in the group of geometric operations is "rotate anticlockwise 90 degrees", and the multiplicative inverse of 3 is 1/3.
Saying "the inverse of 3" to mean 1/3, i.e. its multiplicative inverse, is sloppy but acceptable so long as everyone knows what you mean (and it's fairly common). Saying "the inverse square root" to mean 1/square root is just wrong.