Occurring unexpectedly, unintentionally, or by chance. Music Of or relating to an accidental. Being an animal, especially a bird, that has strayed beyond its normal range. A property, factor, or attribute that is not essential. Music Any of various signs that indicate the alteration of a note by one or two semitones or the cancellation of a previous sign. A note that has been marked with such a sign. An animal that has strayed beyond its normal range.