principal axis has 1 distinct synonym spread across 1 of its 1 sense. 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 line that passes through the center of curvature of a lens so that light is neither reflected nor refracted
“in a normal eye the optic axis is the direction in which objects are seen most distinctly”
Synonyms, antonyms and the broader and narrower relations for principal axis come from Princeton University’s WordNet. Full sources and licences.