weakness has 3 distinct synonyms spread across 2 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. Its one recorded opposite is strength.
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 flaw or weak point
“he was quick to point out his wife's failings”
powerlessness revealed by an inability to act
“in spite of their weakness the group remains active”
Synonyms, antonyms and the broader and narrower relations for weakness come from Princeton University’s WordNet. Full sources and licences.
the property of lacking physical or mental strength; liability to failure under pressure or stress or strain
“his weakness increased as he became older”
the condition of being financially weak
“the weakness of the dollar against the yen”
a penchant for something even though it might not be good for you
“he has a weakness for chocolate”