The Collaborative International Dictionary of English v.0.48:
Essential \Es*sen"tial\ ([e^]s*s[e^]n"sjal), a. [Cf. F.
essentiel. See Essence.]
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
2. Hence, really existing; existent.
Is it true, that thou art but a name,
And no essential thing? --Webster
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
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
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.