parallel has 8 distinct synonyms spread across 4 of its 8 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 perpendicular, oblique.
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.
something having the property of being analogous to something else
an imaginary line around the Earth parallel to the equator
(mathematics) one of a set of parallel geometric figures (parallel lines or planes)
“parallels never meet”
be parallel to
“Their roles are paralleled by ours”
being everywhere equidistant and not intersecting
“parallel lines never converge”
1 of the 8 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for parallel.
Synonyms, antonyms and the broader and narrower relations for parallel come from Princeton University’s WordNet. Full sources and licences.