accidental
adjective
°Happening by chance, or unexpectedly; taking place not according to the usual course of things; casual; fortuitous.
"an accidental visit"
°Nonessential; not necessary belonging; incidental.
"are accidental to a play"
noun
°A property which is not essential; a nonessential; anything happening accidentally.
°(in plural 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.
synonyms: casual, fortuitous, contingent, occasional more» , adventitious, incidental