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

major release

    A release of a piece of software which is not
   merely a revision or a bug fix release but which contains
   substantial changes (e.g., an overhaul of the interface,
   change in compatibility).

   Traditionally, major releases are numbered as X.0; for
   example, WordPerfect 6.0 is a major release, significantly
   different from any previous version; whereas WordPerfect 6.1
   has only minor changes, and is, thus, only a revision.

   See also major delivery.

   (1996-08-04)