late 14c., "non-essential quality," from accidental (adj.). The musical sense is from 1868; so called because they alter the note without essentially changing the key of the passage.

late 14c., "non-essential," from Old French accidentel or directly from Medieval Latin accidentalis, from Latin accidentem "an accident, chance" (see accident). Meaning "outsi [..]

happening by chance or unexpectedly or unintentionally ; "with an inadvertent gesture she swept the vase off the table"; "accidental poisoning"; &am [..]

This an all encompassing term for prefixing a Natural, Double Flat, Flat, Sharp, or Double Sharp to a note. The accidental is primarily used to indicate playing a note outwith the current key. The eff [..]

tsufelik

accidental (pop)

(n) a musical notation that makes a note sharp or flat or natural although that is not part of the key signature(adj) happening by chance or unexpectedly or unintentionally

a sharp, fiat, or natural sign.

Used to describe an event that’s unexpected and unintended.

a music symbol used to alter the pitch of a note, such as a sharp (♯), flat (♭), or natural (♮).

 A notational sign in a score indicating that a specific note is to be played as a flat, sharp, or natural. The most common accidentals (flats and sharps) correspond to the five black notes in each oc [..]

in musical notation, any of the symbols used to raise or lower a pitch by one-two “half steps” that are not placed in the “key signature”. See also: “flat”, “sharp”, “natural”, “half step”, “key signa [..]

A species that has appeared in a given area only a very few times and whose normal range is in another area.

adj. "non-essential," s.v. accidental a. OED. KEY: accidental@adj

adj 1 accidental 1

Something that happens unexpectedly or by chance. (18 Nov 1997)

Not essential; incidental, secondary. from 14th c. (philosophy) Nonessential to something's inherent nature (especially in Aristotelian thought). from 14th c. (music) Adjusted by one or two semi [..]

the sharpening or flattening of a particular note not indicated in the key signature of a piece; the naturalizing of a note that is sharpened or flattened according to the key signature.