I am looking for the source and/or proper definition for the int() function I am looking for the source and/or proper definition for the int() function.
I have searched through the source in Arduino.app, without much success.

The reference gives a terse explanation 

int(x)
 x: a value of any type 
 Converts a value to the int data type


Passing 1, '1' or 49 returns the int 1. I haven't tested other types.

This seems to be an Arduino specific function - it doesn't seem to be in any c/c++ library.
int(x) is not a function but another way to perform an explicit cast of x to an int. It is part of the C++ language itself, that is why you could not find it in a library.

This is not Arduino-specific.

In C and C++ you can cast a value to an int by doing this:

int y = (int) x;


C++ has extended this notation by allowing to do it this way, which is equivalent:

int y = int(x);


You can find this in more details in C++ language reference.