diagonal has 12 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. 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.
(geometry) a straight line connecting any two vertices of a polygon that are not adjacent
a line or cut across a fabric that is not at right angles to a side of the fabric
an oblique line of squares of the same color on a checkerboard
“the bishop moves on the diagonals”
(mathematics) a set of entries in a square matrix running diagonally either from the upper left to lower right entry or running from the upper right to lower left entry
a punctuation mark (/) used to separate related items of information
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 diagonal.
Synonyms, antonyms and the broader and narrower relations for diagonal come from Princeton University’s WordNet. Full sources and licences.