es·sen·tial 
(ĭ-sĕnshəl)
(ĭ-sĕnshəl)adj.
1. Constituting or being part of the nature or essence of something; inherent: "In that era of general good will ... few Americans doubted the essential goodness of their society" (David Halberstam).
2. Fundamentally important or necessary: essential ingredients. See Synonyms at indispensable.
3. Medicine Of, relating to, or being a condition or a disease whose cause is unknown: essential hypertension.
4. 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.
n.
1. Something fundamental.
2. Something necessary or indispensable.
es·sen′ti·ali·ty (-shē-ălĭ-tē), es·sential·ness n.
es·sential·ly adv.