Noun

• S: (n) necessity, essential, requirement, requisite, necessary (anything indispensable) "food and shelter are necessities of life"; "the essentials of the good life"; "allow farmers to buy their requirements under favorable conditions"; "a place where the requisites of water fuel and fodder can be obtained"

Adjective

• S: (adj) essential, indispensable (absolutely necessary; vitally necessary) "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker"
• S: (adj) essential (basic and fundamental) "the essential feature"
• S: (adj) all-important, all important, crucial, essential, of the essence (of the greatest importance) "the all-important subject of disarmament"; "crucial information"; "in chess cool nerves are of the essence"
• S: (adj) essential (being or relating to or containing the essence of a plant etc) "essential oil"
• S: (adj) substantive, essential (defining rights and duties as opposed to giving the rules by which rights and duties are established) "substantive law"