Monopoly is a market, or part of a market, reserved to the exclusive possession of one or more sellers by means...

“Monopoly is a market, or part of a market, reserved to the exclusive possession of one or more sellers by means of the initiation of physical force by the government, or with the sanction of the government. Monopoly exists insofar as the freedom of competition is violated, with the freedom of competition being understood as the absence of the initiation of physical force as the preventive of competition. Where there is no initiation of physical force to violate the freedom of competition, there is no monopoly. The freedom of competition is violated only insofar as individuals are excluded from markets or parts of markets by means of the initiation of physical force. Monopoly is thus a market or part of a market reserved to the exclusive possession of one or more sellers by means of the initiation of physical force. It is thus something imposed upon the market from without—by the government. (Private individuals—gangsters—can initiate force to reserve markets only if the government allows it and thereby sanctions it.)
Thus, monopoly is not something which emerges from the normal operation of the economic system, and which the government must control.”
― George Reisman