Adjective
1. absolutely necessary; vitally necessary;
"essential tools and materials""funds essential to the completion of the project""an indispensable worker"2. basic and fundamental;
"the essential feature"3. of the greatest importance;
"the all-important subject of disarmament""crucial information""in chess cool nerves are of the essence"4. being or relating to or containing the essence of a plant etc;
"essential oil"5. applying to essential legal principles and rules of right;
"substantive law"6. absolutely required and not to be used up or sacrificed