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.