essential (adj.)

mid-14c., "that is such by its essence," from Late Latin essentialis, from essentia "being, essence," abstract noun formed (to translate Greek ousia "being, essence") from essent-, present participle stem of esse "to be," from PIE root *es- "to be." Meaning "pertaining to essence" is from late 14c., that of "constituting the essence of something" is from 1540s; that of "necessary" is from 1520s. Essentials "indispensable elements" is from early 16c. Related: Essentially.

Definitions of essential
essential (adj.)
of the greatest importance;
Synonyms: all-important / all important / crucial / of the essence
essential (adj.)
absolutely necessary; vitally necessary;
essential tools and materials
funds essential to the completion of the project
Synonyms: indispensable
essential (adj.)
basic and fundamental;
the essential feature
essential (adj.)
being or relating to or containing the essence of a plant etc;
essential oil
essential (adj.)
defining rights and duties as opposed to giving the rules by which rights and duties are established;
Synonyms: substantive
essential (n.)
anything indispensable;
the essentials of the good life