burden has 14 distinct synonyms spread across 5 of its 6 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. Its one recorded opposite is unburden.
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 onerous or difficult concern
“the burden of responsibility”
the central idea that is expanded in a document or discourse
weight down with a load
Synonyms, antonyms and the broader and narrower relations for burden come from Princeton University’s WordNet. Full sources and licences.