institution has 15 distinct synonyms spread across 3 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.
an organization founded and united for a specific purpose
an establishment consisting of a building or complex of buildings where an organization for the promotion of some cause is situated
the act of starting something for the first time; introducing something new
“she looked forward to her initiation as an adult”
a hospital for mentally incompetent or unbalanced person
Synonyms, antonyms and the broader and narrower relations for institution come from Princeton University’s WordNet. Full sources and licences.