Cartel

defacto Monopoly

*A cartel is a formal, explicit agreement among competing firms. It is a formal organization of producers and manufacturers that agree to fix prices, marketing, and production.[1] Cartels usually occur in an oligopolistic (Oligopoly) industry, where the number of sellers is small (usually because barriers to entry, most notably startup costs, are high) and the products being traded are usually commodities. Cartel members may agree on such matters as price fixing, total industry output, market shares, allocation of customers, allocation of territories, bid rigging, establishment of common sales agencies, and the division of profits or combination of these. The aim of such collusion (also called the cartel agreement) is to increase individual members' profits by reducing competition.

One can distinguish private cartels from public cartels. In the public cartel a government is involved to enforce the cartel agreement, and the government's sovereignty shields such cartels from legal actions. Inversely, private cartels are subject to legal liability under the Anti-Trust laws now found in nearly every nation of the world. Furthermore, the purpose of private cartels is to benefit only those individuals who constitute it, public cartels, in theory, work to pass on benefits to the populace as a whole.* http://en.wikipedia.org/wiki/Cartel

(I think this can happen on the buy-side, too - defacto Monopsony.)