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