The Collaborative International Dictionary of English v.0.48:

Unessential \Un`es*sen"tial\, a.
   1. Not essential; not of prime importance; not indispensable;
      unimportant. --Addison.
      [1913 Webster]

   2. Void of essence, or real being. [R.] --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unessential \Un`es*sen"tial\, n.
   Something not constituting essence, or something which is not
   of absolute necessity; as, forms are among the unessentials
   of religion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unessential
    adj 1: not basic or fundamental [syn: inessential,
           unessential] [ant: essential]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "unessential":
   accessory, accident, accidental, addendum, addition, additional,
   adjunct, adrift, adscititious, adventitious, aleatory, appendage,
   appurtenance, appurtenant, ascititious, auxiliary, back-burner,
   beside the mark, beside the point, beside the question, casual,
   circumstantial, collateral, conditional, contingency, contingent,
   de trop, dinky, dispensable, excess, expendable, expletive, extra,
   extraneous, extrinsic, fortuitous, gratuitous, happenstance,
   immaterial, impertinent, in excess, inadmissible, inapplicable,
   inapposite, inappreciable, inappropriate, incidental, inconsequent,
   inconsequential, inconsiderable, inessential, inferior,
   insignificant, irrelative, irrelevant, little, mere chance, minor,
   minute, needless, negligible, nihil ad rem, nonessential,
   not at issue, not vital, not-self, occasional, off the subject,
   other, out-of-the-way, parenthetical, petit, pleonastic, prolix,
   provisional, redundant, secondary, small, spare, subsidiary,
   superadded, superaddition, supererogatory, superfluous,
   supervenient, supplement, supplemental, supplementary, tautologic,
   tautologous, technical, to spare, uncalled-for, unimpressive,
   unnecessary, unneeded, unnoteworthy, unrequired, verbose