steep has 12 distinct synonyms spread across 3 of its 6 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 one recorded opposite is gradual.
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.
having a sharp inclination
“the steep attic stairs”
greatly exceeding bounds of reason or moderation
“exorbitant rent”
1 of the 6 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 steep.
Synonyms, antonyms and the broader and narrower relations for steep come from Princeton University’s WordNet. Full sources and licences.