Wikipedia
PhysX is a proprietary realtime physics engine middleware SDK. PhysX was authored at NovodeX, an ETH Zurich spin-off. In 2004 NovodeX was acquired by Ageia, and in February 2008 Ageia was acquired by Nvidia.
The term PhysX can also refer to the PPU expansion card designed by Ageia to accelerate PhysX-enabled video games.
Video games supporting hardware acceleration by PhysX can be accelerated by either a PhysX PPU or a CUDA-enabled GeForce GPU (if it has at least 32 cores and a minimum of 256MB dedicated graphics memory), thus offloading physics calculations from the CPU, allowing it to perform other tasks instead.
Middleware physics engines free game developers from writing own code which implements classical mechanics (Newtonian physics) to do e.g. soft body dynamics. PhysX is one of the handful of physics engines used in the large majority of today's games.
The PhysX engine and SDK are available for Microsoft Windows, OS X, Linux, PlayStation 2, PlayStation 3, PlayStation 4, Xbox 360, Xbox One and the Wii. The PhysX SDK is provided to developers for free for both commercial and non-commercial use on Windows. For Linux, OS X and Android platforms the PhysX SDK is free for educational and non-commercial use. At GDC 2015, Nvidia made PhysX' source code available on GitHub, but requiring registration on the developer.nvidia.com.
The first game to use PhysX was Bet On Soldier: Blood Sport. Nvidia PhysX is part of Nvidia GameWorks.