Explain the role of the open-closed principle in SOLID and what the letter O signifies in this context.

The Open/Closed Principle (OCP) in SOLID is one of the five main principles of object-oriented design. It states that software entities (classes, modules, functions) should be open for extension but closed for modification. This means that the system's behavior can be extended without changing existing code.

The role of this principle in SOLID is to ensure flexibility and robustness of the system against changes. It allows adding new functionality without breaking existing code, reducing the risk of errors and simplifying maintenance.

For example, if new logic needs to be added, it is better to create a new class implementing the required interface rather than changing an existing class, which could affect other parts of the program.