WordNet records no synonym and no antonym for dimorphism. 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.
(chemistry) the property of certain substances that enables them to exist in two distinct crystalline forms
(biology) the existence of two forms of individual within the same animal species (independent of sex differences)
Synonyms, antonyms and the broader and narrower relations for dimorphism come from Princeton University’s WordNet. Full sources and licences.