Inversion of Control (Dependency Inversion)

If class A calls a method of class B, and class B calls a method of class C, then, at compile time, class A will depend on class B, and class B will depend on class C:

Instead, class A can call methods on an abstraction that B implements. B depends on an interface controlled by A at compile time:

This inverts the dependency.
Dependency inversion is what enables dependency injection.