OOP Object-Oriented Programming
A programming style where code is organised around "objects" that combine data and behaviour.
Core concepts: class (template), object (instance of the template), inheritance (one class derives from another), encapsulation (hide internal details), polymorphism (same method, different behaviour per type).
Dominated the 90s and 2000s. Today there are counter-movements (functional programming, composition over inheritance) but OOP is still the default in Java, C#, Python, Ruby.