The Jargon File (version 4.4.7, 29 Dec 2003):

Plan 9
 n.

    In the late 1980s, researchers at Bell Labs (especially Rob Pike of
    Kernighan & Pike fame) got bored with the limitations of UNIX and decided
    to reimplement the entire system. The result was called Plan 9 in ?the Bell
    Labs tradition of selecting names that make marketeers wince.? The
    developers also wished to pay homage to the famous film, ?Plan 9 From Outer
    Space?, considered by some to be the worst movie ever made. The source is
    available for download under open-source terms. The developers and a small
    fan base hang out at comp.os.plan9, where one can occasionally hear ?If you
    want UNIX, you know where to find it?


The Free On-line Dictionary of Computing (30 December 2018):

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.

   (http://plan9.bell-labs.com/plan9dist/).

   (2005-02-15)