What is the opposite of a monopoly?
Solution 1:
There is an infrequently used term Polyopoly also sometime spelled Polypoly.
polyopoly
A market situation in which there are no large sellers but many small ones.
Solution 2:
If it's not a monopoly or an oligopoly, then there are a number of companies competing in the market. So it's a competitive market.
Solution 3:
A perfect competitive market, if it exists.
perfect competition