difficulty has 2 distinct synonyms spread across 2 of its 4 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 ease.
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 factor causing trouble in achieving a positive result or tending to produce a negative result
“serious difficulties were encountered in obtaining a pure reagent”
Synonyms, antonyms and the broader and narrower relations for difficulty come from Princeton University’s WordNet. Full sources and licences.
the quality of being difficult
“they agreed about the difficulty of the climb”