9P (or the Plan 9 Filesystem Protocol or Styx), is a network protocol developed for the Plan 9 from Bell Labs distributed operating system as the means of connecting the components of a Plan 9 system. The file is a central metaphor of Plan 9, and many things are represented as files, including windows, network connections, processes, and almost anything else available in the operating system.
9P encourages caching and also serving of synthetic files (e.g. /proc to represent processes), unlike NFS.
9P was revised for the 4th edition of Plan 9 under the name 9P2000 that contained various fundamental improvements. The latest version of Inferno also uses 9P2000. The Inferno file protocol was originally called Styx, but technically it has always been a variant of 9P.
There is a server implementation of 9P for Unix called u9fs included in the Plan 9 distribution, and a kernel client driver for Linux as part of the v9fs project. 9P (and derivatives) have also found application in embedded environments, such as the Styx on a Brick project