Oligopoly
When several firms control a significant share of market sales, the resulting market structure is called an oligopoly or oligopsony. An oligopoly may engage in collusion, either tacit or overt, and thereby exercise market power. An explicit agreement in an oligopoly to affect market price or output is called a cartel. The behavior of firms in perfect competition or monopoly can be treated as a simple optimization, but an oligopoly requires game theoretic analysis.
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