1.Definition: absolutely necessary; vitally necessary

"Essential tools and materials", "Funds essential to the completion of the project"

Related Adjective(s):indispensable

2.Definition: basic and fundamental

"The essential feature"

3.Definition: being or relating to or containing the essence of a plant etc

"Essential oil"

4.Definition: defining rights and duties as opposed to giving the rules by which rights and duties are established

Related Adjective(s):substantive

5.Definition: of the greatest importance

Related Adjective(s):crucial