Accidental

1. Occurring unexpectedly, unintentionally, or by chance.

2. Music Of or relating to an accidental.

1. A property, factor, or attribute that is not essential.

2. Music

   1. Any of various signs that indicate the alteration of a note by one or two semitones or the cancellation of a previous sign.

   2. A note that has been marked with such a sign.

Synonyms: These adjectives apply to what happens unintentionally. Accidental primarily refers to what occurs by chance: an accidental meeting.
It can also mean subordinate or nonessential: "Poetry is something to which words are the accidental, not by any means the essential form" (Frederick W. Robertson).
Fortuitous stresses chance even more strongly: "the happy combination of fortuitous circumstances" (Sir Walter Scott).
Contingent describes what is possible but uncertain because of unforeseen or uncontrollable factors: "The results of confession were not contingent, they were certain" (George Eliot).
Incidental refers to a minor or unanticipated result or accompaniment: "There is scarcely any practice which is so corrupt as not to produce some incidental good" (Enoch Mellor).
Adventitious applies to something acquired or added externally, sometimes by accident or chance: "The court tries to understand 'whether the young man's misconduct was adventitious or the result of some serious flaw in his character'" (Harry F. Rosenthal).