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"
having existence or being or actuality; "an attempt to refine the existent machinery to make it more efficient"; "much of the beluga caviar existing in the world is found in the Soviet Union and Iran"