Source : Webster's Revised Unabridged Dictionary (1913)

   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.

Source : WordNet®

lemon oil
     n : fragrant yellow oil obtained from the lemon peel