The Collaborative International Dictionary of English v.0.48:

Nonessential \Non`es*sen"tial\, a.
   Not essential.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nonessential \Non`es*sen"tial\, n.
   A thing not essential.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonessential
    adj 1: not of prime or central importance; "nonessential to the
           integral meanings of poetry"- Pubs.MLA [syn:
           incidental, nonessential]
    n 1: anything that is not essential; "they discarded all their
         inessentials" [syn: inessential, nonessential] [ant:
         essential, necessary, necessity, requirement,
         requisite]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "nonessential":
   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, 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, unessential,
   unimpressive, unnecessary, unneeded, unnoteworthy, unrequired,
   verbose