A class that contains a combination of methods from other classes. Mixins encourage code reuse and can be used to avoid the inheritance ambiguity that multiple inheritance can cause (the diamond problem), or to work around lack of support for multiple inheritance in a language. How such a combination is done depends on the language, but it is not by inheritance. If a combination contains all methods of combined classes it is equivalent to multiple inheritance.
Magento UI components, a Less frontend library, use a set of mixins for base elements to ease frontend theme development and customization.