What is the meaning of essential?

noun

• Anything indispensable.
  • usage: "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"
  • synonyms: necessity, requirement, requisite, necessary

adjective

• Absolutely necessary; vitally necessary.
  • usage: "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker"
  • synonyms: indispensable
• Basic and fundamental.
  • usage: "the essential feature"
• Of the greatest importance.
  • usage: "the all-important subject of disarmament"; "crucial information"; "in chess cool nerves are of the essence"
  • synonyms: all-important, all important, crucial, of the essence
• Being or relating to or containing the essence of a plant etc.
  • usage: "essential oil"
• Defining rights and duties as opposed to giving the rules by which rights and duties are established.
  • usage: "substantive law"
  • synonyms: substantive