What does the letter 'O' in SOLID principles stand for?

The letter «O» in the SOLID principles stands for Open/Closed Principle — the principle of openness/closure. 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, reducing the risk of errors and simplifying maintenance.

Example: instead of modifying an existing class, add a new class that implements the required interface, and use polymorphism to extend functionality.