ACCIDENT'AL, adjective

1. Happening by chance, or rather unexpectedly; casual; fortuitous, taking place not according to the usual course of things; opposed to that which is constant, regular, or intended, as an accidental visit.

2. Non-essential; not necessarily belonging to; as songs are accidental to a play.

Accidental colors, are those which depend upon the affections of the eye, in distinction from those which belong to the light itself.

Accidental point, in perspective, is that point in the horizontal line, where the projections of two lines parallel to each other, meet the perspective plane.