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

open source
 n.

    [common; also adj. open-source] Term coined in March 1998 following the
    Mozilla release to describe software distributed in source under licenses
    guaranteeing anybody rights to freely use, modify, and redistribute, the
    code. The intent was to be able to sell the hackers' ways of doing software
    to industry and the mainstream by avoiding the negative connotations (to 
    suits) of the term ?free software?. For discussion of the follow-on
    tactics and their consequences, see the Open Source Initiative site.

    Five years after this term was invented, in 2003, it is worth noting the
    huge shift in assumptions it helped bring about, if only because the hacker
    culture's collective memory of what went before is in some ways blurring.
    Hackers have so completely refocused themselves around the idea and ideal
    of open source that we are beginning to forget that we used to do most of
    our work in closed-source environments. Until the late 1990s open source
    was a sporadic exception that usually had to live on top of a closed-source
    operating system and alongside closed-source tools; entire open-source
    environments like Linux and the *BSD systems didn't even exist in a
    usable form until around 1993 and weren't taken very seriously by anyone
    but a pioneering few until about five years later.


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

open source

    A method and philosophy for software
   licensing and distribution designed to encourage use and
   improvement of software written by volunteers by ensuring that
   anyone can copy the source code and modify it freely.

   The term "open source" is now more widely used than the
   earlier term "free software" (promoted by the Free Software
   Foundation) but has broadly the same meaning - free of
   distribution restrictions, not necessarily free of charge.

   There are various open source licenses available.
   Programmers can choose an appropriate license to use when
   distributing their programs.

   The Open Source Initiative promotes the Open Source
   Definition.

   The Cathedral and the Bazaar

(http://tuxedo.org/~esr/writings/cathedral-bazaar/cathedral-bazaar.html).
   was a seminal paper describing the open source phenomenon.

   Open Sources - O'Reilly book with full text online
   (http://oreilly.com/catalog/opensources/book/perens.html).

   Articles from ZDNet
   (http://zdnet.com/pcmag/features/opensource/).

   (1999-12-29)