If there is only a single seller in a market, we have a (supply) monopoly. The situation when there is a single buyer is called monopsony. If a market involves a single seller and a single buyer, we speak of a bilateral monopoly.
Monopolistic structures can arise through many causes. One is a reduction of unit costs, which can come about through expanded production enabled by increasing internal specialization. The first producer to take advantage of this possibility can ultimately undersell all other producers (Competition) and drive them from the market (“natur…