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