essential

Anything indispensable.

“the essentials of the good life”

Of the greatest importance.

Absolutely necessary; vitally necessary.

“essential tools and materials”

“funds essential to the completion of the project”

Defining rights and duties as opposed to giving the rules by which rights and duties are established.

Basic and fundamental.

“the essential feature”

Being or relating to or containing the essence of a plant etc.

“essential oil”