Core

In libretro / RetroArch terminology, a core is a single emulator implementation packaged as a shared library that conforms to the libretro API. One core emulates one platform (e.g. snes9x emulates the SNES, mGBA emulates the Game Boy Advance). The frontend (RetroArch, EmulatorJS, etc.) loads cores at runtime, so adding a new platform to a frontend is a matter of dropping in a new core file rather than rewriting the host application.