Necessary. / Very important; of high importance. / (biology) Necessary for survival but not synthesized by the organism, thus needing to be ingested. / Being in the basic form; showing its essence.

A necessary ingredient. / A fundamental ingredient.

in essence

plural of essential

(by extension) The information required to have a basic understanding of a topic.

(philosophy) The view that objects have properties that are essential to them.

The condition of being essential; a basic set of essential traits; being / An essential thing.

plural of essentialism

The condition of being essential; essentiality

present participle of essentialize