WordNet records no synonym and no antonym for monopoly. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
exclusive control or possession of something
“They have no monopoly on intelligence”
Synonyms, antonyms and the broader and narrower relations for monopoly come from Princeton University’s WordNet. Full sources and licences.
a board game in which players try to gain a monopoly on real estate as pieces advance around the board according to the throw of a die