Es·sen·tial a.
 1. Belonging to the essence, or that which makes an object, or class of objects, what it is.
    Majestic as the voice sometimes became, there was forever in it an essential character of plaintiveness.   --Hawthorne.
 2. Hence, really existing; existent.
 Is it true, that thou art but a name,
 And no essential thing?   --Webster (1623).
 3. Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.
 Judgment's more essential to a general
 Than courage.   --Denham.
    How to live? -- that is the essential question for us.   --H. Spencer.
 4. Containing the essence or characteristic portion of a substance, as of a plant; highly rectified; pure; hence, unmixed; as, an essential oil. “Mine own essential horror.”
 5. Mus. Necessary; indispensable; -- said of those tones which constitute a chord, in distinction from ornamental or passing tones.
 6. Med. Idiopathic; independent of other diseases.
 Essential character Biol., the prominent characteristics which serve to distinguish one genus, species, etc., from another.
 Essential disease, Essential fever Med., one that is not dependent on another.
 Essential oils Chem., a class of volatile oils, extracted from plants, fruits, or flowers, having each its characteristic odor, and hot burning taste. They are used in essences, perfumery, etc., and include many varieties of compounds; as lemon oil is a terpene, oil of bitter almonds an aldehyde, oil of wintergreen an ethereal salt, etc.; -- called also volatile oils in distinction from the fixed or nonvolatile.