When a dominant position is achieved in an industry or sector, to the exclusion of viable competitors.
These tend to lead to increased pricing along with a reduction in quality. It is something that free trade nations discourage.
Many countries have laws designed to prevent the ongoing success of a monopoly. Throughout history, governments broke them up, forcing competition into the industry. The main reason is because consumers lack choice when monopolies exist.
Characteristics
The five characteristics of a monopoly: