A market situation where a few buyers exert a disproportionate influence on the market.
So, potentially many sellers, but few buyers. This is the opposite of an oligopoly, which has many buyers but few sellers.
An example of an oligopsony would be a situation where only a few large companies are using a particular raw material (tobacco, timber, etc.) and that material is being provided by numerous small suppliers. The buyers have many options while the sellers have few and are therefore at a disadvantage.
Etymology: Greek, fron oligoi, few + opsonia, purchase of provisions, shopping. Apparently originated in 1942; that's the date in my dictionary, and the earliest reference in Google books.