A de facto monopoly is a monopoly that was not created by government. It is most often used in contrast to de jure monopoly, which is one that is protected from competition by government action.
In a free market without government intervention this kind of monopoly is theoretically unobtainable for any extended amount of time. A de facto monopoly is only able to be achieved by providing a far demanded product at all times compared to the competition, and even then there would not be a 100% market share.
See also