horizontal 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 2 recorded opposites are vertical, inclined.
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 that is oriented horizontally
Synonyms, antonyms and the broader and narrower relations for horizontal come from Princeton University’s WordNet. Full sources and licences.