What does the Open/Closed Principle mean?

Short answer: The Open/Closed Principle (OCP) states that: Software entities (classes, modules, functions) should be open for extension but closed for modification. This means you should be able to add new behavior to a class without changing its existing code, which helps avoid breaking existing functionality.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.