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

The letter "I" in SOLID principles stands for Interface Segregation Principle — the principle of separating interfaces.

The essence of this principle is that clients should not depend on interfaces they do not use. It is better to create several specialized interfaces rather than one large universal interface. This helps make the code more flexible and understandable, as well as easier to maintain and extend.

Example: instead of one interface with methods for reading and writing, it is better to create two separate interfaces — for reading and for writing, so that a class can implement only the functionality it needs.