A 2D physics engine is a software framework that simulates physical interactions in a two-dimensional environment. It allows developers to create realistic movements and behaviors for objects, such as bouncing, sliding, and colliding. These engines use mathematical models to calculate forces, velocities, and accelerations, making it easier to design games and simulations. Common features of 2D physics engines include collision detection, rigid body dynamics, and constraints. Popular examples include Box2D and Chipmunk, which are widely used in game development. By integrating a physics engine, developers can enhance the realism and interactivity of their projects, improving user experience.