Source : Free On-Line Dictionary of Computing

de facto standard
     
        A widespread consensus on a particular product or {protocol}
        which has not been ratified by any official {standard}s 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)