Source : Free On-Line Dictionary of Computing

Plan 9
     
         (Named after the classically bad,
        exceptionally low-budget SF film "Plan 9 from Outer Space") An
        {operating system} developed at {Bell Labs} by many
        researchers previously intimately involved with {Unix}.
     
        Plan 9 is superficially Unix-like but features far finer
        control over the {name-space} (on a per-process basis) and is
        inherently distributed and scalable.
     
        Plan 9 is divided according to service functions.  CPU servers
        concentrate computing power into large multiprocessors; file
        servers provide repositories for storage and terminals give
        each user of the system a dedicated computer with bitmap
        screen and mouse on which to run a window system.  The sharing
        of computing and file storage services provides a sense of
        community for a group of programmers, amortises costs and
        centralises and hence simplifies management and
        administration.
     
        The pieces communicate by a single protocol, built above a
        reliable data transport layer offered by an appropriate
        network, that defines each service as a rooted tree of files.
        Even for services not usually considered as files, the unified
        design permits some simplification.  Each process has a local
        file name space that contains attachments to all services the
        process is using and thereby to the files in those services.
        One of the most important jobs of a terminal is to support its
        user's customised view of the entire system as represented by
        the services visible in the name space.
     
        {Documentation (ftp://plan9.att.com/dist/plan9doc/)} (an FTP
        server running Plan 9).