essential

adjective

• 1absolutely necessary; extremely important: [with infinitive]:it is essential to keep up-to-date records fibre is an essential ingredient of our diet
• [attributive] fundamental or central to the nature of something or someone:the essential weakness of the plaintiff’s case

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

• 3 Medicine (of a disease) with no known external stimulus or cause; idiopathic:essential hypertension

noun

• a thing that is absolutely necessary:we only had the bare essentials in the way of equipment
• (essentials) the fundamental elements or characteristics of something:he was quick to grasp the essentials of an opponent’s argument