Opposite of monopoly
What is the opposite (antonym) of the word “Monopoly”?
You are probably looking for a word that means competitive market. In fact, people have suggested that you actually use these 2 words as an antonym for monopoly1. Clear and easily understood.
However if you really want a single word antonym, you could consider using Polypoly (or the alternative spelling Polyopoly). It means: “The presence of a large number of relatively small buyers and sellers, none of whom can influence the price of commodities2”. This sounds like “competitive market” however this word is no longer in common usage, so use with care.
If monopoly is where one seller dominates a market, what do you call a market dominated by one buyer? There is a word for that: Monopsony3. An example would be the Department of Defence for local suppliers. It is an opposite but maybe not an antonym. However it may be the opposite you are looking for.