eccentricity has no recorded synonym. 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 concentricity.
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.
strange and unconventional behavior
(geometry) a ratio describing the shape of a conic section; the ratio of the distance between the foci to the length of the major axis
“a circle is an ellipse with zero eccentricity”
a circularity that has a different center or deviates from a circular path
Synonyms, antonyms and the broader and narrower relations for eccentricity come from Princeton University’s WordNet. Full sources and licences.