Cartel
Plain definition
A cartel is a formal agreement among competing firms to fix prices, limit output, or divide markets so they behave like a single monopolist instead of rivals.
In economics
Cartels are the most direct form of collusion and are illegal in most economies under antitrust law. They are most likely to survive when there are few firms, similar costs, homogeneous products, and effective punishment of cheaters.
Example
The classic example is OPEC, whose member countries agree on oil production quotas to influence world prices; when members exceed their quotas, the cartel weakens.
Common confusion
People often use 'cartel' for any big or powerful company, but a cartel is specifically a group of independent firms that collude; a single large monopoly is not a cartel.