Which concept describes different objects responding differently to the same message?

Polymorphism is a fundamental concept in object-oriented programming where different objects can respond to the same method call or message in various ways. This characteristic allows for flexibility and reusability of code, as a single interface can operate on different types of objects. For instance, if you have a method that invokes a draw function, polymorphism allows different shapes, such as circles and squares, to implement their own specific version of the draw method. Thus, when the draw message is sent, the shape object will respond according to its own defined behavior, which enhances the dynamism and includes extensibility in a software system.

The other concepts mentioned, such as inheritance, encapsulation, and aggregation, relate to different aspects of object-oriented design. Inheritance refers to deriving new classes from existing ones, encapsulation focuses on keeping the internal state of an object hidden from the outside, and aggregation describes a "whole-part" relationship between objects. While these concepts are essential to understanding object-oriented design, they do not capture the essence of multiple objects responding distinctly to the same message like polymorphism does.