Game data file used by games developed with Source, a first-person shooter (FPS) game engine developed by Valve; stores a navigation mesh, which consists of points and connected lines that make up potential traversal paths for artificial intelligence (AI) non-player characters (NPCs), or "bots;" used for real-time navigation and movement for AI computer players.

The original nodegraph is created using the Level Designer component of the Source engine SDK, which is called the Valve Hammer Editor. Once complete, an AIN file is automatically compiled by the Source engine when the game is loaded. The compiled AIN file is saved to the \maps\graphs\ directory within the game installation directory. NOTE: The Valve Source engine is used for games such as Half-Life 2 and Team Fortress 2.