Monopoly: A situation where a single company or group owns all or nearly all of the market for a given type of product or service.
Trust: A large business with significant market power, historically typified by massive industries such as railroads and telegraph companies that were once regulated under U.S. law through anti-trust laws.
Robber Barons: Negative term for the powerful 19th-century American businessmen who made fortunes by monopolizing huge industries through the formation of trusts, engaging in unethical business practices, exploiting workers, and paying little heed to their customers or competition.