essential

adj. necessary, needed, fundamental, impossible to live without

Essential or essentials can refer to:An essential property, which defines an entity as being a particular type of entity. See also essentialism, the philosophical view that an entity must have certain characteristics in order to belong to a certain defined group, and its counterpart, non-essentialism.The Essential series of compilation albums released by Sony BMGEither of two companies named Essential Records.Essential, an album by the Divinyls.Essential, an album by Amanda Lear.Essentials, an album by The Cars.Essentials, an album by Travis Tritt.The Essentials of Fire Fighting, a firefighter training manual published by the International Fire Service Training Association and Fire Protection PublicationsThe Essential line of reprints from Marvel Comics.
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n. essentialness, necessariness, indispensability, vitalness

(a.)
Necessary; indispensable; -- said of those tones which constitute a chord, in distinction from ornamental or passing tones.
  

(a.)
Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.
  

(a.)
Idiopathic; independent of other diseases.
  

(a.)
Hence, really existing; existent.
  

(a.)
Containing the essence or characteristic portion of a substance, as of a plant; highly rectified; pure; hence, unmixed; as, an essential oil.
  

(a.)
Belonging to the essence, or that which makes an object, or class of objects, what it is.
  

As in essential hypertension. A hallowed term meaning "We don't know the cause." Synonymous with idiopathic.