it is a class enclosed in the scope of another class. For illustration:
// Example: Nested class
Nested classes are helpful for organizing code and controlling dependencies and access. Nested classes obey access rules as other parts of class do; thus, in Example, if Nested Class is public then any code may name it as OuterClass::NestedClass. Frequently nested classes have private implementation details, and are thus made private; in Example, if NestedClass is private, then just OuterClass's members and friends can employ NestedClass. While you instantiate like outer class, it won't instantiate inside class.