essential has 10 distinct synonyms spread across 4 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 inessential.
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.
basic and fundamental
“the essential feature”
of the greatest importance
“the all-important subject of disarmament”
defining rights and duties as opposed to giving the rules by which rights and duties are established
“substantive law”
anything indispensable
“food and shelter are necessities of life”
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 essential.
Synonyms, antonyms and the broader and narrower relations for essential come from Princeton University’s WordNet. Full sources and licences.