The Collaborative International Dictionary of English v.0.48:

Wintergreen \Win"ter*green`\, n. (Bot.)
   A plant which keeps its leaves green through the winter.
   [1913 Webster]

   Note: In England, the name wintergreen is applied to the
         species of Pyrola which in America are called
         English wintergreen, and shin leaf (see Shin leaf,
         under Shin.) In America, the name wintergreen is
         given to Gaultheria procumbens, a low evergreen
         aromatic plant with oval leaves clustered at the top of
         a short stem, and bearing small white flowers followed
         by red berries; -- called also checkerberry, and
         sometimes, though improperly, partridge berry.
         [1913 Webster]

   Chickweed wintergreen, a low perennial primulaceous herb
      (Trientalis Americana); -- also called star flower.

   Flowering wintergreen, a low plant (Polygala paucifolia)
      with leaves somewhat like those of the wintergreen
      (Gaultheria), and bearing a few showy, rose-purple
      blossoms.

   oil of wintergreen, An aromatic oil, consisting almost
      entirely of methyl salicylate (CH3CO.O.C6H4.OH),
      obtained by distillation of an extract of the wintergreen
      (Gaultheria procumbens); it can also be obtained from
      some other plants. It is used as a flavoring agent for
      tooth powders and pastes, sometimes combined with menthol
      or eucalyptus. It is called also oil of teaberry, oil
      of partridgeberry, and oil of gaultheria.

   Spotted wintergreen, a low evergreen plant (Chimaphila
      maculata) with ovate, white-spotted leaves.
      [1913 Webster + PJC]

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.
      [1913 Webster]

            Majestic as the voice sometimes became, there was
            forever in it an essential character of
            plaintiveness.                        --Hawthorne.
      [1913 Webster]

   2. Hence, really existing; existent.
      [1913 Webster]

            Is it true, that thou art but a name,
            And no essential thing?               --Webster
                                                  (1623).
      [1913 Webster]

   3. Important in the highest degree; indispensable to the
      attainment of an object; indispensably necessary.
      [1913 Webster]

            Judgment's more essential to a general
            Than courage.                         --Denham.
      [1913 Webster]

            How to live? -- that is the essential question for
            us.                                   --H. Spencer.
      [1913 Webster]

   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.
      [1913 Webster]

   5. (Mus.) Necessary; indispensable; -- said of those tones
      which constitute a chord, in distinction from ornamental
      or passing tones.
      [1913 Webster]

   6. (Med.) Idiopathic; independent of other diseases.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oil of wintergreen
    n 1: oil or flavoring obtained from the creeping wintergreen or
         teaberry plant [syn: wintergreen oil, oil of
         wintergreen]