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

Version 7
 /vee' se?vn/, n.

    The first widely distributed version of Unix, released unsupported by
    Bell Labs in 1978. The term is used adjectivally to describe Unix features
    and programs that date from that release, and are thus guaranteed to be
    present and portable in all Unix versions (this was the standard gauge of
    portability before the POSIX and IEEE 1003 standards). Note that this usage
    does not derive from the release being the ?seventh version of Unix?;
    research Unix at Bell Labs has traditionally been numbered according to
    the edition of the associated documentation. Indeed, only the
    widely-distributed Sixth and Seventh Editions are widely known as V[67];
    the OS that might today be known as ?V10? is instead known in full as ?
    Tenth Edition Research Unix? or just ?Tenth Edition? for short. For this
    reason, ?V7? is often read by cognoscenti as ?Seventh Edition?. See BSD,
    Unix. Some old-timers impatient with commercialization and kernel bloat
    still maintain that V7 was the Last True Unix.


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

Version 7

    (V7) The unsupported release of Unix
   ancestral to all current commercial versions.  Brian
   Kernighan announced the release of V7 in summer 1979, at the
   Unix User's Group meeting in Toronto.

   Before the release of the POSIX/SVID standards, V7's
   features were often treated as a Unix portability baseline.
   Some old-timers impatient with commercialisation and kernel
   bloat still maintain that V7 was the Last True Unix.

   See BSD, USG Unix, System V.

   [Jargon File]

   (1996-05-22)