accidental

Short definition from Wiktionary

Adjective

1. 1
   Pertaining to accident and not essence; thus, inessential; incidental; secondary.
2. 2
   Pertaining to accident and not essence; thus, inessential; incidental; secondary: Nonessential to something's inherent nature (especially in Aristotelian thought).
3. 3
   Adjusted by one or two semitones, in temporary departure from the key signature.
4. 4
   Occurring sometimes, by chance; occasional.
5. 5
   Happening by chance, or unexpectedly; taking place not according to the usual course of things; by accident, unintentional.
6. 6
   Being a double point with two distinct tangent planes in 4-dimensional projective space.

Noun

1. 1
   A property which is not essential; a nonessential; anything happening accidentally.
2. 2
   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.
3. 3
   A sharp, flat, or natural, occurring not at the commencement of a piece of music as the signature, but before a particular note.
4. 4
   Part of a text that has a mainly structural purpose, such as spelling, punctuation, or capitalization.