Definitions of essential:

• noun:   anything indispensable

  Example: "The essentials of the good life"

• adjective:   absolutely necessary; vitally necessary

  Example: "Essential tools and materials"

• adjective:   being or relating to or containing the essence of a plant etc

  Example: "Essential oil"

• adjective:   basic and fundamental

  Example: "The essential feature"

• adjective:   absolutely required and not to be used up or sacrificed
• adjective:   applying to essential legal principles and rules of right
• adjective:   of the greatest importance