es·sen·tial / iˈsenshəl/ • adj. 1. absolutely necessary; extremely important: it is essential to keep up-to-date records. ∎ fundamental or central to the nature of something or someone: the essential weakness of the plaintiff's case. ∎ Biochem. (of an amino acid or fatty acid) required for normal growth but not synthesized in the body and therefore necessary in the diet.2. Med. (of a disease) with no known external stimulus or cause; idiopathic: essential hypertension.• n. (usu. essentials) a thing that is absolutely necessary: we had only the bare essentials in the way of gear. ∎ (essentials) the fundamental elements or characteristics of something: he was quick to grasp the essentials of an opponent's argument.DERIVATIVES: es·sen·ti·al·i·ty / iˌsenshēˈalitē/ n.es·sen·tial·ness n.