Inverse function

Graphical definition:

the reflection of the curve across the line y = x. Can also be thought of as switching the y and x axes.

Methodological definition:

substitute y for x and x for y.

Practical definition:

if f(x) is any function, and g is its inverse, then g(f(x)) = x and f(g(x)) = x. In other words, an inverse function undoes what the original function does.