Observer Interface (IObserver):?

Short answer: This interface defines the Update() method, which will be implemented by concrete observers. Each observer will receive updates from the subject when the state changes. public interface IObserver

Example code

{ void Update(string news); }

Real-world example (ShopNest)

When order status changes, observers update email, SMS, and analytics without the Order class knowing each one.

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.