The Collaborative International Dictionary of English v.0.48:

Accidental \Ac`ci*den"tal\, a. [Cf. F. accidentel, earlier
   accidental.]
   1. Happening by chance, or unexpectedly; taking place not
      according to the usual course of things; casual;
      fortuitous; as, an accidental visit.
      [1913 Webster]

   2. Nonessential; not necessary belonging; incidental; as, are
      accidental to a play.
      [1913 Webster]

   Accidental chords (Mus.), those which contain one or more
      tones foreign to their proper harmony.

   Accidental colors (Opt.), colors depending on the
      hypersensibility of the retina of the eye for
      complementary colors. They are purely subjective
      sensations of color which often result from the
      contemplation of actually colored bodies.

   Accidental point (Persp.), the point in which a right line,
      drawn from the eye, parallel to a given right line, cuts
      the perspective plane; so called to distinguish it from
      the principal point, or point of view, where a line drawn
      from the eye perpendicular to the perspective plane meets
      this plane.

   Accidental lights (Paint.), secondary lights; effects of
      light other than ordinary daylight, such as the rays of
      the sun darting through a cloud, or between the leaves of
      trees; the effect of moonlight, candlelight, or burning
      bodies. --Fairholt.
      [1913 Webster]

   Syn: Casual; fortuitous; contingent; occasional;
        adventitious.

   Usage: Accidental, Incidental, Casual, Fortuitous,
          Contingent. We speak of a thing as accidental when
          it falls out as by chance, and not in the regular
          course of things; as, an accidental meeting, an
          accidental advantage, etc. We call a thing incidental
          when it falls, as it were, into some regular course of
          things, but is secondary, and forms no essential part
          thereof; as, an incremental remark, an incidental
          evil, an incidental benefit. We speak of a thing as
          casual, when it falls out or happens, as it were, by
          mere chance, without being prearranged or
          premeditated; as, a casual remark or encounter; a
          casual observer. An idea of the unimportant is
          attached to what is casual. Fortuitous is applied to
          what occurs without any known cause, and in opposition
          to what has been foreseen; as, a fortuitous concourse
          of atoms. We call a thing contingent when it is such
          that, considered in itself, it may or may not happen,
          but is dependent for its existence on something else;
          as, the time of my coming will be contingent on
          intelligence yet to be received.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Accidental \Ac`ci*den"tal\, n.
   1. A property which is not essential; a nonessential;
      anything happening accidentally.
      [1913 Webster]

            He conceived it just that accidentals . . . should
            sink with the substance of the accusation. --Fuller.
      [1913 Webster]

   2. pl. (Paint.) Those fortuitous effects produced by luminous
      rays falling on certain objects so that some parts stand
      forth in abnormal brightness and other parts are cast into
      a deep shadow.
      [1913 Webster]

   3. (Mus.) A sharp, flat, or natural, occurring not at the
      commencement of a piece of music as the signature, but
      before a particular note.
      [1913 Webster]

WordNet (r) 3.0 (2006):

accidental
    adj 1: happening by chance or unexpectedly or unintentionally ;
           "with an inadvertent gesture she swept the vase off the
           table"; "accidental poisoning"; "an accidental shooting"
           [syn: accidental, inadvertent]
    n 1: a musical notation that makes a note sharp or flat or
         natural although that is not part of the key signature

Moby Thesaurus II by Grady Ward, 1.0:

139 Moby Thesaurus words for "accidental":
   accessory, accident, accidentally, accompanying, addendum,
   addition, additional, adjunct, ado, adscititious, adventitious,
   afloat, afoot, aleatory, appendage, appurtenance, appurtenant,
   ascititious, auxiliary, breve, by-the-way, casual, casually,
   causeless, chance, chancy, circumstantial, coincident,
   coincidental, collateral, conditional, contingency, contingent,
   crotchet, current, demisemiquaver, dependent, destinal, dicey,
   doing, dominant, dominant note, double whole note, eighth note,
   enharmonic, enharmonic note, eventuating, extra, fatal, fatidic,
   flat, fluky, fortuitous, fortuitously, going on, half note,
   happening, happenstance, hemidemisemiquaver, iffy, in hand,
   in the wind, inadvertent, incidental, indeterminate, inessential,
   lucky, mere chance, minim, musical note, natural, nonessential,
   not-self, note, occasional, occurring, odd, on, on foot, ongoing,
   other, parenthetical, passing, patent note, prevailing, prevalent,
   provisional, quarter note, quaver, random, report, responding note,
   resultant, risky, secondary, semibreve, semiquaver, serendipitous,
   shaped note, sharp, sixteenth note, sixty-fourth note, spiccato,
   staccato, subsidiary, superadded, superaddition, superfluous,
   supervenient, supplement, supplemental, supplementary,
   sustained note, taking place, tercet, thirty-second note, tone,
   triplet, unanticipated, uncalculated, uncaused, under way,
   undesigned, undetermined, unessential, unexpected, unforeseeable,
   unforeseen, unintended, unintentional, unlooked-for, unlucky,
   unmeant, unplanned, unpredictable, unpremeditated, unpurposed,
   unwitting, whole note