The Collaborative International Dictionary of English v.0.48:

Volatile \Vol"a*tile\, a. [F. volatil, L. volatilis, fr. volare
   to fly, perhaps akin to velox swift, E. velocity. Cf.
   Volley.]
   1. Passing through the air on wings, or by the buoyant force
      of the atmosphere; flying; having the power to fly. [Obs.]
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   2. Capable of wasting away, or of easily passing into the
      aeriform state; subject to evaporation.
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   Note: Substances which affect the smell with pungent or
         fragrant odors, as musk, hartshorn, and essential oils,
         are called volatile substances, because they waste away
         on exposure to the atmosphere. Alcohol and ether are
         called volatile liquids for a similar reason, and
         because they easily pass into the state of vapor on the
         application of heat. On the contrary, gold is a fixed
         substance, because it does not suffer waste, even when
         exposed to the heat of a furnace; and oils are called
         fixed when they do not evaporate on simple exposure to
         the atmosphere.
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   3. Fig.: Light-hearted; easily affected by circumstances;
      airy; lively; hence, changeable; fickle; as, a volatile
      temper.
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            You are as giddy and volatile as ever. --Swift.
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   Volatile alkali. (Old Chem.) See under Alkali.

   Volatile liniment, a liniment composed of sweet oil and
      ammonia, so called from the readiness with which the
      latter evaporates.

   Volatile oils. (Chem.) See Essential oils, under
      Essential.
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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.
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            Majestic as the voice sometimes became, there was
            forever in it an essential character of
            plaintiveness.                        --Hawthorne.
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   2. Hence, really existing; existent.
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            Is it true, that thou art but a name,
            And no essential thing?               --Webster
                                                  (1623).
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   3. Important in the highest degree; indispensable to the
      attainment of an object; indispensably necessary.
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            Judgment's more essential to a general
            Than courage.                         --Denham.
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            How to live? -- that is the essential question for
            us.                                   --H. Spencer.
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   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." --Ford.
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   5. (Mus.) Necessary; indispensable; -- said of those tones
      which constitute a chord, in distinction from ornamental
      or passing tones.
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   6. (Med.) Idiopathic; independent of other diseases.
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   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.
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