[ac.ci.den.tal] adj (14c) 1: arising from extrinsic causes: incidental, nonessential
2. a: occurring unexpectedly or by chance b: happening without intent or through carelessness and often with unfortunate results -- ac.ci.den.tal.ly also ac.ci.dent.ly adv -- ac.ci.den.tal.ness n syn accidental, fortuitous, casual, contingent mean not amenable to planning or prediction. accidental stresses chance "any resemblance to actual persons is entirely accidental". fortuitous so strongly suggests chance that it often connotes entire absence of cause "a series of fortuitous events". casual stresses lack of real or apparent premeditation or intent "a casual encounter with a stranger". contingent suggests possibility of happening but stresses uncertainty and dependence on other future events for existence or occurrence "the contingent effects of the proposed law".
[2]accidental n (1651) 1: a nonessential property
2. a: a note foreign to a key indicated by a signature b: a prefixed sign indicating an accidental