The visibility of an identifier is that region of the program source code from which legal access can be made to the identifier's associated object. 

Scope and visibility usually coincide, though there are circumstances under which an object becomes temporarily hidden by the appearance of a duplicate identifier: the object still exists but the original identifier cannot be used to access it until the scope of the duplicate identifier is ended.

Again, special rules apply to hidden class names and class member names: C++ operators allow hidden identifiers to be accessed under certain conditions