accidental

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Occurring unexpectedly, unintentionally, or by chance.
2. adj. Music Of or relating to an accidental.
3. n. A property, factor, or attribute that is not essential.
4. n. Music Any of various signs that indicate the alteration of a note by one or two semitones or the cancellation of a previous sign.
5. n. Music A note that has been marked with such a sign.

Century Dictionary and Cyclopedia

1. Happening by chance or accident, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous; unintentional: as, an accidental meeting.
2. Non-essential; not necessarily belonging to the subject; adventitious: as, songs are accidental to a play.
3. Synonyms Accidental, Chance, Casual, Fortuitous, Incidental, Contingent. The first four are the words most commonly used to express occurrence without expectation or design. Accidental is the most common, and expresses that which happens outside of the regular course of events. Chance has about the same force as accidental, but it is not used predicatively. There is a tendency to desynonymize accidental and casual, so as to make the former apply to events that are of more consequence: as, an accidental fall; a casual remark. As to actual connection with the main course of events, casual is the word most opposed to incidental; the connection of what is incidental is real and necessary, but secondary: as, an incidental benefit or evil. An incidental remark is a real part of a discussion; a casual remark is not. Fortuitous is rather a learned word, not applicable in many cases where accidental or even casual could be used; perhaps through its resemblance to fortunate, it is rarely if ever used when speaking of that which is unfavorable or undesired; thus, it would not be proper to speak of a fortuitous shipwreck. It is chiefly used with the more abstract words: as, fortuitous events; a fortuitous resemblance. That which is contingent is dependent upon something else for its happening: as, his recovery is contingent upon the continuance of mild weather. See occasional.
4. n. Anything happening, occurring, or appearing accidentally, or as if accidentally; a casualty. Specifically— In music, a sign occurring in the course of a piece of music and altering the pitch of the note before which it is placed from the pitch indicated by the signature, or restoring it to the latter after it has undergone such alteration. There are five such signs: the sharp (♯), double sharp (×), flat (♭), double flat (d♭), and natural (♮). The sharp raises the pitch a half step, the double sharp a whole step; the flat lowers the pitch a half step, the double flat a whole step; the natural annuls the effect of a previous sharp or flat occurring either in the signature or as an accidental. The effect of an accidental is usually limited to the bar in which it occurs. In medicine, tissue resulting from morbid action: chiefly employed in this sense by French writers, but adopted by some English authors.
5. n. An unessential property; a mere adjunct or circumstance.

Wiktionary

1. adj. Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous
2. adj. Nonessential; not necessary belonging; incidental
3. adj. music Foreign to the key signature or a proper harmony.
4. n. A property which is not essential; a nonessential; anything happening accidentally.
5. n. painting Those fortuitous effects produced by luminous rays falling on certain objects so that some parts stand forth in abnormal brightness and other parts are cast into a deep shadow.
6. n. music A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.

GNU Webster's 1913

1. adj. Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous.
2. adj. Nonessential; not necessary belonging; incidental.
3. n. A property which is not essential; a nonessential; anything happening accidentally.
4. n. (Paint.) Those fortuitous effects produced by luminous rays falling on certain objects so that some parts stand forth in abnormal brightness and other parts are cast into a deep shadow.
5. n. (Mus.) A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.

WordNet 3.0

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