WordNet records no synonym and no antonym for ecchymosis. 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.
the purple or black-and-blue area resulting from a bruise
the escape of blood from ruptured blood vessels into the surrounding tissue to form a purple or black-and-blue spot on the skin
Synonyms, antonyms and the broader and narrower relations for ecchymosis come from Princeton University’s WordNet. Full sources and licences.