A trust or corporate trust is a large grouping of business interests with significant market power, which may be embodied as a corporation or as a group of corporations that cooperate with one another in various ways. These ways can include constituting a trade association, owning stock in one another, constituting a corporate group, or combinations thereof. The term trust is often used in a historical sense to refer to monopolies or near-monopolies in the United States during the Second Industrial Revolution in the 19th century and early 20th century. The use of corporate trusts during this period is the historical reason for the name "antitrust law".

A monopoly, as described by Irving Fisher, is a market with the "absence of competition", creating a situation where a specific person or enterprise is the only supplier of a particular thing. This contrasts with a monopsony which relates to a single entity's control of a market to purchase a good or service, and with oligopoly and duopoly which consists of a few sellers dominating a market. Monopolies are thus characterised by a lack of economic competition to produce the good or service, a lack of viable substitute goods, and the possibility of a high monopoly price well above the seller's marginal cost that leads to a high monopoly profit. The verb monopolise or monopolize refers to the process by which a company gains the ability to raise prices or exclude competitors. In economics, a monopoly is a single seller. In law, a monopoly is a business entity that has significant market power, that is, the power to charge overly high prices, which is associated with unfair price raises. Although monopolies may be big businesses, size is not a characteristic of a monopoly. A small business may still have the power to raise prices in a small industry.