libretro is an API that exposes generic audio/video/input callbacks. A frontend
for libretro (such as RetroArch) handles video output, audio output, input and
application lifecycle. A libretro core written in portable C or C++ can run
seamlessly on many platforms with very little to no porting effort.
While RetroArch is the reference frontend for libretro, several other projects
have used the libretro interface to include support for emulators and/or game
engines. libretro is completely open and free for anyone to use.
See also: retroarch-freeworld.