WordNet records no synonym and no antonym for colloquium. 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.
an academic meeting or seminar usually led by a different lecturer and on a different topic at each meeting
an address to an academic meeting or seminar
Synonyms, antonyms and the broader and narrower relations for colloquium come from Princeton University’s WordNet. Full sources and licences.