understanding has 7 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.
the cognitive condition of someone who understands
“he has virtually no understanding of social cause and effect”
the statement (oral or written) of an exchange of promises
“they had an agreement that they would not interfere in each other's business”
an inclination to support or be loyal to or to agree with an opinion
“his sympathies were always with the underdog”
1 of the 5 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for understanding.
Synonyms, antonyms and the broader and narrower relations for understanding come from Princeton University’s WordNet. Full sources and licences.