In an oligopoly, a few powerful merchants or companies control the entire market. If you and three friends buy up all the toilet paper factories in the world, you will have a toilet paper oligopoly.
Oligopoly comes from the ancient Greek oligo-, for "few," and pole, for "merchant," but the term wasn't invented until the late 19th century. In an economic oligopoly, only a few producers control the supply of something (like steel, cars, or clown suits), and each one has a lot of influence over prices. Oligopoly might remind you of monopoly, which is when a single producer controls the whole market.