What are classes called that provide additional behavior to methods but are not intended to be instantiated themselves?

Classes that provide additional behavior to methods but are not intended to be instantiated themselves are known as mixin classes. Mixins allow code reuse within classes in a way that promotes modularity. They provide methods that can be added to other classes to extend functionality without forming a strict hierarchy as seen in traditional inheritance.

For instance, instead of a class hierarchy where a class inherits from another to gain behavior, a mixin can be used to supplement classes with specific methods relevant to their needs. This approach enables a more flexible design, allowing for the composition of behaviors from multiple sources without creating a rigid class structure.

In contrast, derived classes are specifically intended to create new classes based on existing ones, thereby inheriting properties and methods and are meant to be instantiated. Base classes serve as foundational classes for derived classes and are usually instantiated directly. Inheritance classes isn’t a standard term used in object-oriented programming, which makes it less relevant in this context.

Thus, mixin classes serve their unique role in enhancing functionality without the limitation of instantiation, making them the correct answer for this question.