1. absolutely necessary; indispensable.
2. pertaining to or constituting the essence of a thing.
3. noting or containing an essence of a plant, drug, etc.
4. being such by its very nature or in the highest sense; natural; spontaneous: essential
5. not associated with an underlying disease: essential hypertension.
6. a basic, indispensable, or necessary element; chief point.
[1300–50; Middle English essencial < Medieval Latin essenciālis, for Late Latin essentiālis]
syn: essential, inherent, intrinsic refer to that which is in the natural composition of a thing. essential suggeststhat which is in the very essence or constitution of a thing: Quiet is essential in a public library. inherent meansinborn or fixed from the beginning as a permanent quality or constituent of a thing: properties inherent in iron.intrinsic implies belonging to the nature of a thing itself and comprised within it, without regard to externalconsiderations or accidentally added properties: the intrinsic value of diamonds. See also necessary.
Source: Random House Kernerman Webster’s College Dictionary, © 2010.