Monopoly is indeed not technically the right term, but it's not necessarily a monopoly itself that causes antitrust issues, it's the business practices that are

Monopoly is indeed not technically the right term, but it's not necessarily a monopoly itself that causes antitrust issues, it's the business practices that are enabled by it. A large player in a market that uses business practices to capture and hold more of that market by erecting barriers of entry are definitely sailing into antitrust waters. In essence, any practice that helps compete in the market by other ways than increasing value to the consumer is suspect. Incidentally, this is a double edges sword, as this is the interpretation of the antitrust laws that has enabled the rise of so many monopoly-like companies in the last few decades. (e.g. Amazon: we're increasing value to customers because we can offer lower prices if we're larger)