【Accidental】 , 【incidental】 and 【adventitious】 all mean not being part of the real or essential nature of something.
【Accidental】 retains its basic notion of chance occurrence but may also imply nonessential character.
【Incidental】 suggests a real, sometimes a designed, relationship but one which is secondary and nonessential.
【Adventitious】 implies a lack of essential relationships and may suggest casual addition or irrelevance.