Explain what an abstract class in object-oriented programming is.

An abstract class in object-oriented programming is a class that is not intended to be instantiated directly. It serves as a base template for other classes and can contain:

• Abstract methods — declared but not implemented in the abstract class itself, and must be implemented in subclasses.
• Regular methods with implementations, which can be used or overridden in inheriting classes.

An abstract class helps define a common interface and behavior for a group of related classes, while allowing for specific implementations in subclasses.

Example in Java:

abstract class Animal {
    abstract void makeSound(); // abstract method

    void sleep() {
        System.out.println("Sleeping...");
    }
}

class Dog extends Animal {
    void makeSound() {
        System.out.println("Bark");
    }
}

Here, Animal cannot be instantiated directly, but Dog, which implements the abstract method, can be.