In Economics, monopoly exists when a specific individual or enterprise has sufficient control over a particular product or service to determine significantly the terms on which other individuals shall have access to it. Monopolies are thus characterized by a lack of economic competition for the good or service that they provide and a lack of viable substitute goods. Alternatively , it may be used as a verb or adjective to refer to the process by which a firm gains persistently greater market share than what is expected under perfect competition. The latter usage of the term is invoked in the theory of monopolistic competition.

It is often argued that monopolies tend to become less efficient and innovative over time, becoming "complacent giants", because they do not have to be efficient or innovative to compete in the marketplace. Sometimes this very loss of efficiency can raise a potential competitor's value enough to overcome market entry barriers, or provide incentive for research and investment into new alternatives. The theory of contestable markets argues that in some circumstances monopolies are forced to behave as if there were competition because of the risk of losing their monopoly to new entrants. This is likely to happen where a market's barriers to entry are low. It might also be because of the availability in the longer term of substitutes in other markets. For example, a canal monopoly, while worth a great deal in the late eighteenth century United Kingdom,was worth much less in the late nineteenth century because of the introduction of railways as a substitute.

Some argue that it can be good to allow a firm to attempt to monopolize a market, since practices such as dumping can benefit consumers in the short term; and once the firm grows too big, it can be dealt with via regulation. When monopolies are not broken through the open market, often a government will step in, either to regulate the monopoly, turn it into a publicly owned monopoly, or forcibly break it up (see Antitrust law). Public utilities, often being natural monopolies and less susceptible to efficient breakup, are often strongly regulated or publicly owned. AT&T and Standard Oil are debatable examples of the breakup of a private monopoly. When AT&T was broken up into the "Baby Bell" components, MCI, Sprint, and other companies were able to compete effectively in the long distance phone market and began to take phone traffic from the less efficient AT&T.