essential

Concise Oxford English Dictionary © 2008 Oxford University Press:
essential /ɪˈsɛnʃ(ə)l/

▶adjective

• 1 absolutely necessary; extremely important.

• 2 central to the nature of something; fundamental: the essential weakness of the plaintiff's case.

• 3 (of an amino acid or fatty acid) required for normal growth but not synthesized in the body and therefore necessary in the diet.

• 4 Medicine (of a disease) with no known external stimulus or cause; idiopathic.

▶noun (essentials)

• 1 things that are absolutely necessary.

• 2 the fundamental elements of something.

– derivatives
essentiality noun,
essentially adverb,
essentialness noun.

– origin ME (in the sense ‘in the highest degree’): from late L. essentialis, from L. essentia (see essence).