perpendicular has 5 distinct synonyms spread across 3 of its 7 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 oblique, parallel.
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 straight line at right angles to another line
a Gothic style in 14th and 15th century England; characterized by vertical lines and a four-centered (Tudor) arch and fan vaulting
a cord from which a metal weight is suspended pointing directly to the earth's center of gravity; used to determine the vertical from a given point
1 of the 7 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 perpendicular.
Synonyms, antonyms and the broader and narrower relations for perpendicular come from Princeton University’s WordNet. Full sources and licences.