/
ɪˈsɛnʃ(ə)l/
adj.
    - absolutely necessary; extremely important.
 - central to the nature of something; fundamental:
 the essential weakness of the plaintiff's case.
 - (of an amino acid or fatty acid) required for normal growth but not synthesized in the body and therefore necessary in the diet.
 - Medicine  (of a disease) with no known external stimulus or cause; idiopathic.
 
n.
 
(
essentials)
  
- things that are absolutely necessary.
 - the fundamental elements of something.
 
Derivative
- essentiality n.
  - essentially adv.
  - essentialness n.
 
Etymology
ME (in the sense ‘in the highest degree’): from late L. essentialis, from L. essentia (see essence).