AMAZON has caused no small controversy of late by refusing to accept presale orders on books to be released by the publisher Hachette and by understocking Hachette’s titles. These punitive maneuvers, which follow a dispute between Amazon and Hachette about e-book contracts, have led to significant delays in shipments of Hachette’s books to Amazon’s customers.

If you are wondering why Amazon would subject its customers to this inconvenience and wish to understand what’s really happening between Amazon and Hachette — and, indeed, all the major book publishers — you need to know the meaning of the word monopsony.

The Supreme Court justice Sonia Sotomayor, when sitting on a lower court, once described monopsony as the “mirror image” of monopoly. Unlike a monopoly, which occurs when a seller of goods has the power to unlawfully raise prices of what it sells, a monopsony occurs when a buyer of goods has the power to unlawfully lower the prices of what it buys. Each violates antitrust laws: As the Supreme Court has long recognized, they both result in a misallocation of resources that harms consumers and distorts markets.

Take the e-book market, dominated by Amazon, which buys what a federal court once found to be 90 percent of all e-books sold in the United States. The monopsony power of Amazon, which has a current market share of 65 percent of all online book units, digital and print, is not just theoretical; it’s real and formidable. When Macmillan, the fifth largest book publisher, displeased Amazon in 2010 by proposing certain changes in business terms, Amazon exercised what has been described as its “nuclear option”: It promptly deleted the “buy” buttons in the Amazon online store for all of Macmillan’s books. In an instant, Macmillan’s entire business was in jeopardy.