A group of companies that agree, often quietly, to sell something at the same high price, or to share the market, so they do not really compete. The point is to make more money by stopping a fight over price. Economists and newspapers use it in this business sense; it sounds serious, and it is not a compliment. Though actually, the cartel you hear about most in the news now is a drug cartel: a powerful criminal group that controls the trade in illegal drugs. That is a later use of the same idea — a tight group holding a market. There is also an older political sense: a written agreement between sides, and even, hundreds of years ago, a written challenge to a duel. Students almost always meet the business or crime meaning.