If the Function f : A-> B is both one to one and onto i.e bijective ,then we can find a function g: B-> A
such that
g(y)=x when y=f(x). The function g is called the inverse of f and is denoted as f^{-1}. The function f(x) is called invertible function

Another definition of Invertible function
A Function f : A-> B is invertible if we can find a function g: B- > A such that
fog=y gof=x

Example
A set A is defined as A={a,b,c}
Let f: A-> A be the function defined as are
(1) f={(a,a),(b,b),(c,c)}
(2) f={(a,b),(b,a),(c,c)}
(3) f={(a,c),(b,c),(c,a)}
Find if all these function defined are invertible Solutions
(1) The neccesary condition for invertibleness is one on one and onto
This function is clearly one on one and onto,so it is invertible
(2) This function is clearly one on one and onto,so it is invertible
(3) This function is not one on one and neither onto,so it is not invertible