happening by chance or accident; not planned; unexpected: an accidental meeting.
nonessential; incidental; subsidiary: accidental benefits.
Music. relating to or indicating sharps, flats, or naturals.
a nonessential or subsidiary circumstance, characteristic, or feature.
Music. a sign placed before a note indicating a chromatic alteration of its pitch.

1350–1400; Middle English < Medieval Latin accidentālis. See accident, -al1

accidentally, accidently, adverb
accidentalness, accidentality, noun
nonaccidental, adjective, noun
nonaccidentally, adverb
nonaccidentalness, noun
preaccidental, adjective
preaccidentally, adverb
pseudoaccidental, adjective
pseudoaccidentally, adverb
quasi-accidental, adjective
quasi-accidentally, adverb
unaccidental, adjective
unaccidentally, adverb

1. unintentional, unforeseen. Accidental, casual, fortuitous all describe something outside the usual course of events. Accidental implies occurring unexpectedly or by chance: an accidental blow. Casual describes a passing event of slight importance: a casual reference. Fortuitous is applied to events occurring without known cause, often of a fortunate or favorable nature: a fortuitous shower of meteors. It often also implies good luck or good fortune: a fortuitous choice leading to rapid advancement.

1. planned, contrived. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
accidental (ˌæksɪˈdɛntəl)
1.  occurring by chance, unexpectedly, or unintentionally
2.  nonessential; incidental
3.  music denoting sharps, flats, or naturals that are not in the key signature of a piece
4.  logic (of a property) not essential; contingent
5.  an incidental, nonessential, or supplementary circumstance, factor, or attribute
6.  music a symbol denoting a sharp, flat, or natural that is not a part of the key signature

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Encyclopedia Britannica


in music, sign placed immediately to the left of (or above) a note to show that the note must be changed in pitch. A sharp () raises a note by a semitone; a flat () lowers it by a semitone; a natural () restores it to the original pitch. Double sharps () and double flats () indicate that the note is raised or lowered by two semitones. Sharps or flats that are placed at the beginning of a musical staff, called a key signature, indicate the tonality, or key, of the music and are not considered accidentals.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The cause of this population decline is unknown, but it's likely a combination
  of accidental and intentional overfishing.
Any lenience toward a particular diving outfit was accidental on my part.
Three of the other incidents involved accidental cuts.
But you have to stay on the program, even when the grade-grubbers and
  accidental plagiarists start to line up outside your office.
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