To hide (or protect) a global variable, put two underscores (__) before the name. Protecting a global variable means that it will be hidden and can not be examined or shown. However, the behavior is different depending upon the context.
This example will not print the variable because it is protected:
Show(::__xyz);
This example returns a NULL value for the protected variable:
::a = Name Expr( ::__xyz );
This example does return a value for the protected variable:
Show( Eval( ::__xyz ) );