lightness has 8 distinct synonyms spread across 5 of its 6 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 2 recorded opposites are heaviness, darkness.
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.
the property of being comparatively small in weight
“the lightness of balsa wood”
the gracefulness of a person or animal that is quick and nimble
the visual effect of illumination on objects or scenes as created in pictures
“he could paint the lightest light and the darkest dark”
Synonyms, antonyms and the broader and narrower relations for lightness come from Princeton University’s WordNet. Full sources and licences.