What do we call the unreasonable restriction of business activities that results from conspiracy among members of a trade?

The answer is restraint of trade. This means any action which tends to bound trade, sales and transportation in regional commerce or has a considerable impact on interstate commerce. Antitrust law forbids most of these kinds of practices. The central antitrust law is the Sherman Act. This act prevents trusts from making restraints on trade or commerce and reducing competition. In 1890, the Congress approved the Sherman Antitrust Act. The Sherman Act targets to eliminate restraints on trade and competition. States also have laws contrary to restraints of trade that have strictly local impact.