concern has 19 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. Its one recorded opposite is unconcern.
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.
something that interests you because it is important or affects you
“the safety of the ship is the captain's concern”
a feeling of sympathy for someone or something
“She felt strong concern for those less fortunate”
a commercial or industrial enterprise and the people who constitute it
“he bought his brother's business”
Synonyms, antonyms and the broader and narrower relations for concern come from Princeton University’s WordNet. Full sources and licences.