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

de facto standard

   A widespread consensus on a particular product or protocol
   which has not been ratified by any official standards body,
   such as ISO, but which nevertheless has a large market
   share.

   The archetypal example of a de facto standard is the IBM PC
   which, despite is many glaring technical deficiencies, has
   gained such a large share of the personal computer market
   that it is now popular simply because it is popular and
   therefore enjoys fierce competition in pricing and software
   development.

   (1994-10-27)