Final answer:
In object-oriented programming, the process of deriving new classes from existing ones is known as inheritance, which is essential for promoting code reusability and establishing a hierarchical class structure.
Step-by-step explanation:
In object-oriented programming, when you derive new classes from existing classes, this is known as inheritance. Inheritance allows for the creation of a subclass that inherits attributes and behaviors (methods) from a parent or base class, while also having the ability to introduce its own unique properties and methods or override the existing ones. It's a fundamental concept that enables code reusability and the creation of a hierarchical structure of classes. For example, if you have a base class Vehicle, you could inherit characteristics and functions to create more specific classes like Car or Truck, which would have all vehicles' common features plus their own specific attributes.