This is opposite of a monopoly. In a monopoly, there is a single seller and lots of buyers, allowing the seller to control the market.
In a monopsony, there is a single buyer, which allows the buyer to control the market.
For example, if you manufacture trigger mechanisms for nuclear warheads, you only have one buyer – the federal government. Even if you were able to sell to other, friendly governments, your own government would exercise considerable power over the terms of that sale. There’s just not a whole lot of buyers for what you’re selling.
Why I Looked It Up
In a book about centrally-planned economies:
The problem for the planners was that a hierarchical system that had been well-suited to the activity of total war – an activity characterized by monopsony, as the state is the sole buyer […]
Counter-intuitively, this presents monopsony as a good thing for manufacturers. The book made the book that when war ends, manufacturers move from having to make a sole buyer happy to having to make the entire market happy. When you just have a single buyer, there’s certainly risk involved, but you can mold your business around that single buyers, which has some advantages.