An abstract extensible class that you use in conjunction with a foundation class to define subclasses via multiple inheritance. Mixin classes define sets of attributes that would not be useful alone, but can be used to customize other classes to serve particular purposes.

a class that is designed to provide operations and/or a fragment of interface via inheritance

a class whose super class is specified in an abstract way, by specifying the minimum set of fields and methods it should have

an interesting concept, and is often a good design choice