What does the letter D in the SOLID principles mean in software development?

The D in SOLID principles stands for Dependency Inversion Principle. It states that:

• High-level modules should not depend on low-level modules. Both should depend on abstractions.
• Abstractions should not depend on details. Details should depend on abstractions.

This reduces coupling in the code and increases its flexibility and testability. For example, instead of a class directly creating an instance of another class, it should work through an interface or protocol, and the specific implementation should be passed from outside (via constructor or setter).