Accidental

1. n. A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.
2. a. Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous; as, an accidental visit.
3. a. Nonessential; not necessary belonging; incidental; as, are accidental to a play.
4. n. A property which is not essential; a nonessential; anything happening accidentally.
5. n. 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.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.