essential in WordNet English dictionary

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

1. 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"
   necessity  essential  requirement  requisite  necessary