demonstration has 5 distinct synonyms spread across 4 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
a show or display; the act of presenting something to sight or view
“the presentation of new data”
a show of military force or preparedness
“he confused the enemy with feints and demonstrations”
a public display of group feelings (usually of a political nature)
“there were violent demonstrations against the war”
proof by a process of argument or a series of proposition proving an asserted conclusion
a visual presentation showing how something works
“the lecture was accompanied by dramatic demonstrations”
Synonyms, antonyms and the broader and narrower relations for demonstration come from Princeton University’s WordNet. Full sources and licences.