\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.
2. Nonessential; not necessary belonging; incidental; as, are
   accidental to a play.
{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.
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.

\Ac`ci*den"tal\, n.
1. A property which is not essential; a nonessential;
   anything happening accidentally.
         He conceived it just that accidentals . . . should
         sink with the substance of the accusation. --Fuller.
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.
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.