A monopsony is a monopoly by a buyer rather than a seller. In other words, a monopsony is a market structure that has only one buyer of a good or service.
An example of a monopsony is Major League Baseball. Another example is a powerful union in a closed shop.
If a monopsony wishes to buy an additional good or service, then it must raise the price for all of the goods to that level, in order to buy the additional one.