A name can be hidden by an explicit declaration of that same name in a nested declarative region or derived class ([class.member.lookup]).
A class name or enumeration name can be hidden by the name of a variable, data member, function, or enumerator declared in the same scope. If a class or enumeration name and a variable, data member, function, or enumerator are declared in the same scope (in any order) with the same name, the class or enumeration name is hidden wherever the variable, data member, function, or enumerator name is visible.
In a member function definition, the declaration of a name at block scope hides the declaration of a member of the class with the same name; see [basic.scope.class]. The declaration of a member in a derived class hides the declaration of a member of a base class of the same name; see [class.member.lookup].
During the lookup of a name qualified by a namespace name, declarations that would otherwise be made visible by a using-directive can be hidden by declarations with the same name in the namespace containing the using-directive; see [namespace.qual].