absolutely required and not to be used up or sacrificed
absolutely necessary; vitally necessary; "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker"
being or relating to or containing the essence of a plant etc; "essential oil"
necessarily or demonstrably true; "demonstrable truths"
capable of being demonstrated or proved; "obvious lies"; "a demonstrable lack of concern for the general welfare"; "practical truth provable to all men"- Walter Bagehot