A necessary ingredient. / A fundamental ingredient.

(biochemistry) One of the nine naturally occurring amino acids that the human body cannot synthesize, and so must be provided by dietary protein.

The inherent nature of a thing or idea. / (philosophy) The true nature of anything, not accidental or illusory. / Constituent substance. / A being; especially, a purely spiritual being.

in essence

A volatile oil, used to make perfumes and flavourings, especially one having the characteristic odour of the plant from which it is obtained.

(biochemistry) any fatty acid required for human metabolism that cannot be synthesized by the body and must be present in the diet; originally designated as vitamin F

A major industrial city in North Rhine-Westphalia, in western Germany. / A surname from German.

(nutrition) A compound that must be obtained from food for survival

Important, indispensable, crucial.

(set phrase) Time is an essential consideration (in regard to an action or response); haste is necessary.