Medieval name for the tritone. The tritone is an interval which consists of three whole tones, and is said to add a distinctively dark, unsettling, evil feel to a musical composition, because of the dissonant arrangement of notes. On a bass, you could play it like this:
Before you rush over to your axe, play it and wait for the Prince of Darkness to appear, you should bear in mind that medieval compositions are quite a lot different in sound from modern ones, and that you have probably already heard it quite often. Black Sabbath uses it a lot, so does Korn, Nine Inch Nails, and just about any metal, punk, hardcore and hard rock band on the planet. Slayer even named an album (Diabolus in Musica) after what it was called in the Middle Ages, and uses the interval throughout the album. It's also used in some contemporary jazz, atmospheric tunes, movie soundtracks, and lots more, because it is good for adding tension to a composition. Even some classical composers have used it, including such evil long-haired Satan worshippers as Beethoven, Wagner and Bach.
But that is now. In the Middle Ages, things were different, and the Catholic Church had the sort of control over music the RIAA has today. It was widely believed that even listening to the tritone could cause the hapless listener to be possessed by Satan, and therefore it was prohibited by law. Medieval composers risked being burned at the stake for including it in their pieces.