easiness has 4 distinct synonyms spread across 2 of its 3 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.
a feeling of refreshing tranquility and an absence of tension or worry
“the easiness we feel when sleeping”
freedom from difficulty or hardship or effort
“he rose through the ranks with apparent ease”
Synonyms, antonyms and the broader and narrower relations for easiness come from Princeton University’s WordNet. Full sources and licences.
the quality of being easy in behavior or style
“there was an easiness between them”