But that is only one type of monopoly. The basic meaning of monopoly is that there is no entry into the industry to expand the supply. Therefore the existing firms, whether one or several, are the single source of that product. To enter the industry, one cannot obtain some input resources and expand the output. Instead, one must buy out one of the existing firms.
Some monopolies are legal monopolies, it being illegal to enter the industry to compete.
For example, the delivery of first class mail in the United States is an absolute monopoly of the United States Postal Service. In other cases, the government requires a permit to operate in the industry, and limits the number of permits. The firms then have monopoly power, since they can limit production and raise the price above what would be the competitive price. In other cases, monopolies are legally protected by copyrights and patents which presumably encourage inventions and creative work. Landownership is inherently a monopoly, since it is impossible to manufacture or import space, or to create more material natural resources such as oil and minerals.