A monopoly is the exclusive ownership or control of something. If your sister claims a monopoly over the television during the Tom Cruise movie marathon, you would be wise not to touch that remote.
In economics, a monopoly specifically means the domination of a market by one owner or seller. Think of the board game Monopoly. You’re trying to own all the properties so that you have a monopoly over the board and the other players have to pay you basically anywhere they land. You might recognize the prefix mono, meaning “one." Add it to the Greek word polein which means “sell,” and there you have it — one seller.