es·sen·tial
adj.
1. Constituting or being part of the essence of something; inherent.
2. Basic or indispensable; necessary: essential ingredients. See Synonyms at indispensable.
3. Medicine Of, relating to, or being a dysfunctional condition or a disease whose cause is unknown: essential hypertension.
4. Biochemistry Being a substance that is required for normal functioning but cannot be synthesized by the body and therefore must be included in the diet: essential amino acids.
n.
1. Something fundamental.
2. Something necessary or indispensable.