interest has 11 distinct synonyms spread across 8 of its 10 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 bore.
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 sense of concern with and curiosity about someone or something
“an interest in music”
the power of attracting or holding one's attention (because it is unusual or exciting etc.)
“they said nothing of great interest”
a fixed charge for borrowing money; usually a percentage of the amount borrowed
“how much interest do you pay on your mortgage?”
(law) a right or legal share of something; a financial involvement with something
“they have interests all over the world”
(usually plural) a social group whose members control some field of activity and who have common aims
“the iron interests stepped up production”
Synonyms, antonyms and the broader and narrower relations for interest come from Princeton University’s WordNet. Full sources and licences.