Anything related to the use of Dependency Injection (DI)/IoC containers on a Sitecore solution. This also includes the baked-in Microsoft Dependency Resolver included natively in Sitecore versions 8.2 and above.
Dependency injection (DI) is a technique for achieving loose coupling between objects and their collaborators, or dependencies. Without dependency injection, a class must directly instantiate collaborator or use static references. With dependency injection, the objects a class need are provided for it.