chair has 10 distinct synonyms spread across 5 of its 7 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 seat for one person, with a support for the back
“he put his coat over the back of the chair and sat down”
the position of professor
“he was awarded an endowed chair in economics”
the officer who presides at the meetings of an organization
“address your remarks to the chairperson”
an instrument of execution by electrocution; resembles an ordinary seat for one person
“the murderer was sentenced to die in the chair”
a particular seat in an orchestra
“he is second chair violin”
Synonyms, antonyms and the broader and narrower relations for chair come from Princeton University’s WordNet. Full sources and licences.