DirectX 9 is a set of application programming interfaces (APIs) developed by Microsoft that allows software developers to create multimedia applications, particularly video games, for the Windows operating system. Released in 2002, it provides tools for rendering 2D and 3D graphics, handling audio, and managing input from devices like keyboards and game controllers. One of the key features of DirectX 9 is its support for advanced graphics techniques, such as pixel shaders and vertex shaders, which enhance visual effects and realism in games. This version also introduced improved performance and compatibility, making it a popular choice for game developers during its time.