What role does the "abstract" keyword play in class definitions?

The "abstract" keyword in class definitions indicates that the class cannot be instantiated directly, serving as a blueprint for other subclasses. When a class is declared as abstract, it may contain one or more abstract methods—methods that are declared without implementation. This signifies that any subclass derived from this abstract class must provide an implementation for these abstract methods in order to be instantiated.

By defining methods as abstract, the developer establishes an expected behavior for subclasses while promoting a form of polymorphism. This design encourages adherence to a specific interface or contract without dictating how that functionality must be achieved. Thus, when a class is marked as abstract, it compels any subclass to implement its abstract methods if it wishes to create instances of itself, fostering a clear structure and enforcing necessary functionality in derived classes.