Constituting or being part of the essence of something; inherent. Basic or indispensable; necessary: essential ingredients. See Synonyms at indispensable. Medicine Of, relating to, or being a dysfunctional condition or a disease whose cause is unknown: essential hypertension. 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. Something fundamental. Something necessary or indispensable.