The Cournot assumption is a fundamental concept in oligopoly market analysis. It is an economic model that assumes that firms in an oligopoly market compete based on the quantity of output they produce. This model was developed by French economist Antoine Augustin Cournot in the 19th century.

The Cournot assumption assumes that there are only a few firms in the market, and they are interdependent in their decision-making. Each firm assumes that its rivals will not change their output levels in response to its own output decision. This assumption is based on the idea that firms in an oligopoly market have a limited understanding of their competitors' production costs and demand conditions.

Under the Cournot assumption, each firm chooses its output level based on its expectation of how its rivals will react. This leads to a situation where each firm produces less than it would in a perfectly competitive market, but more than it would in a monopoly market. This is because each firm takes into account the impact of its output decision on the market price and the output decisions of its rivals.

The Cournot assumption has been widely used in economic analysis to model oligopoly markets. It provides a useful framework for understanding the behavior of firms in these markets and the impact of their decisions on market outcomes. However, it is important to note that the Cournot assumption is just one of many assumptions that can be made about oligopoly markets, and its validity depends on the specific market conditions being analyzed.