A physics engine is a software component that simulates physical systems in a virtual environment. It calculates how objects interact based on the laws of physics, such as gravity, friction, and collision. This allows for realistic movements and behaviors in video games, simulations, and animations. These engines are essential for creating immersive experiences, as they help developers design how objects fall, bounce, or collide. Popular examples of physics engines include Box2D, Unity's PhysX, and Havok. By using these tools, creators can ensure that their virtual worlds behave in a way that feels natural to users.