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.
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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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Essential \Es*sen"tial\ ([e^]s*s[e^]n"sjal), n.
   1. Existence; being. [Obs.] --Milton.
      [1913 Webster]

   2. That which is essential; first or constituent principle;
      as, the essentials of religion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

essential
    adj 1: absolutely necessary; vitally necessary; "essential tools
           and materials"; "funds essential to the completion of the
           project"; "an indispensable worker" [syn: essential,
           indispensable]
    2: basic and fundamental; "the essential feature" [ant:
       inessential, unessential]
    3: of the greatest importance; "the all-important subject of
       disarmament"; "crucial information"; "in chess cool nerves
       are of the essence" [syn: all-important(a), all
       important(p), crucial, essential, of the essence(p)]
    4: being or relating to or containing the essence of a plant
       etc; "essential oil"
    5: defining rights and duties as opposed to giving the rules by
       which rights and duties are established; "substantive law"
       [syn: substantive, essential] [ant: adjective,
       procedural]
    n 1: anything indispensable; "food and shelter are necessities
         of life"; "the essentials of the good life"; "allow farmers
         to buy their requirements under favorable conditions"; "a
         place where the requisites of water fuel and fodder can be
         obtained" [syn: necessity, essential, requirement,
         requisite, necessary] [ant: inessential,
         nonessential]

Moby Thesaurus II by Grady Ward, 1.0:

183 Moby Thesaurus words for "essential":
   ab ovo, austere, axiom, bare, bare necessities, basal, basic,
   basilar, bedrock, bench mark, brass tacks, call for, called for,
   capital, cardinal, cardinal point, center, central, chaste, chief,
   chief thing, climax, condition, congenital, connate, constituent,
   constitutional, constitutive, core, cornerstone, crisis,
   critical point, crux, deep-seated, demand, demand for,
   desideration, desideratum, element, elemental, elementary, elixir,
   essence, essential facts, essential matter, essentials, flower,
   focal, focus, foremost, fundamental, gist, gravamen, great point,
   gut, heart, high point, homely, homespun, homogeneous, hypostasis,
   imperative, important, important thing, inborn, inbred, indicated,
   indispensable, indivisible, innate, inner essence, intrinsic,
   irreducible, irreductible, irreplaceable, issue, kernel, key,
   keystone, landmark, leading, life-and-death, life-or-death, main,
   main point, main thing, marrow, material, material point, meat,
   mere, milestone, monolithic, must, must item, necessaries,
   necessary, necessities, necessitous, necessity, need, need for,
   needed, needful, nub, nucleus, nuts and bolts, occasion,
   of a piece, of the essence, of vital importance, original,
   part and parcel, pith, pivot, plain, postulate, prerequirement,
   prerequisite, primal, primary, prime, primitive, principal,
   principle, pure, pure and simple, quid, quiddity, quintessence,
   quintessential, radical, real issue, required, requirement,
   requisite, requisition, right-hand, rudiment, rudimentary,
   salient point, sap, severe, simon-pure, simple, sine qua non,
   single, soul, spare, spirit, stark, stuff, substance, substantial,
   substantive, substantive point, the bottom line, the data,
   the details, the dope, the facts, the information, the necessary,
   the needful, the nitty-gritty, the particulars, the picture,
   the point, the scoop, the score, the specifics, the whole story,
   turning point, unadorned, uncluttered, underlying, undifferenced,
   undifferentiated, unforgoable, uniform, vital, want, wanted