Accidentals, passing tones, lots of names. But just take a common chord in the key of C, the C7. It's C - E - G - Bb. I guess if I were reading this in sheet music, I would consider it an accidental, but it's a very common one, and doesn't necessarily take you out of the key of C.
BTW, the interval between the E and the Bb is called a Tritone, and it's comprised of 3 whole steps. If E were the root the Bb would be the diminished 5th or augmented 4th. I've read that in the early days of standardized Western music, the tritone was considered "The Devil's interval" and was forbidden by the Catholic Church. I've also read that's an exaggeration. Devil's interval or not, I like it.
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