accidental
US /ˌæk.səˈden.t̬əl/
UK /ˌæk.səˈden.t̬əl/
1.
happening by chance or unexpectedly
:
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Their meeting was purely accidental.
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He made an accidental discovery.
1.
a musical note that is not part of the key signature but is introduced by a sharp, flat, or natural sign
:
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The composer used an accidental to temporarily alter the pitch of the note.
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You need to play that F sharp, it's an accidental.