"Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal."
Such is the very first sentence of the Sherman Antitrust Act.
Getting convictions can be a pain in the ass, and there are many things that could be violations but are not usually treated as such in practice, but the statute in theory is very broad, and the general principle is that actions which seriously harm competition in a market are unlawful. The acts do not have to be specifically enumerated in advance.