Definition of diabolus in musica in US English:

diabolus in musica

(also diabolos in musica)

noun

Music
  • Tritone.

Origin

Mid 19th century. From post-classical Latin diabolus in musica the devil in music, more fully mi contra fa, est diabolus in musica ‘mi’ against ‘fa’ is the devil in music, after German Mi contra Fa ist der Teuffel in der Musica; so called as the interval was regarded as unstable and rejected as a consonance by most theorists.