An association of firms, producing similar commodities, formed in order to keep up prices by restricting output is called?

The term you're looking for is "a cartel". A cartel is a group of companies that produce similar products or services, and they work together to control the supply of their products or services in order to keep prices high. They do this by limiting the amount of goods or services they produce, which creates an artificial scarcity that drives up prices. Essentially, it's a way for companies to collude and fix prices in their industry, which is often illegal and can harm consumers by reducing competition.