accidental

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Occurring unexpectedly, unintentionally, or by chance.
• adjective Music Of or relating to an accidental.
• adjective Being an animal, especially a bird, that has strayed beyond its normal range.
• noun A property, factor, or attribute that is not essential.
• noun Any of various signs that indicate the alteration of a note by one or two semitones or the cancellation of a previous sign.
• noun A note that has been marked with such a sign.
• noun An animal that has strayed beyond its normal range.

from The Century Dictionary.

• Happening by chance or accident, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous; unintentional: as, an accidental meeting.
• Non-essential; not necessarily belonging to the subject; adventitious: as, songs are accidental to a play.
• 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.
• noun Anything happening, occurring, or appearing accidentally, or as if accidentally; a casualty.
• noun An unessential property; a mere adjunct or circumstance.

from the GNU version of the Collaborative International Dictionary of English.

• noun A property which is not essential; a nonessential; anything happening accidentally.
• noun (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.
• noun (Mus.) A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.
• adjective Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous.
• adjective Nonessential; not necessary belonging; incidental.
• adjective (Mus.) those which contain one or more tones foreign to their proper harmony.
• adjective (Opt.) colors depending on the hypersensibility of the retina of the eye for complementary colors. They are purely subjective sensations of color which often result from the contemplation of actually colored bodies.
• adjective (Persp.) the point in which a right line, drawn from the eye, parallel to a given right line, cuts the perspective plane; so called to distinguish it from the principal point, or point of view, where a line drawn from the eye perpendicular to the perspective plane meets this plane.
• adjective (Paint.) secondary lights; effects of light other than ordinary daylight, such as the rays of the sun darting through a cloud, or between the leaves of trees; the effect of moonlight, candlelight, or burning bodies.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous
• adjective Nonessential; not necessary belonging; incidental
• adjective music Foreign to the key signature or a proper harmony.
• noun A property which is not essential; a nonessential; anything happening accidentally.
• noun 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.
• noun music A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective happening by chance or unexpectedly or unintentionally
• noun a musical notation that makes a note sharp or flat or natural although that is not part of the key signature