tritone

In music, the interval of the diminished fifth, exactly half the octave, and considered in the Middle Ages to be the moral antithesis of the octave's perfect consonance, or ‘diabolus in musica’ (devil in music). Its prominence during the late 19th and early 20th centuries helped to undermine the remaining foundations of tonality, as in Debussy's Prélude à l'après-midi d'un faune (1894).

The alternative term ‘mi contra fa’ refers to B in the dominant key of G (do-re-mi) in relation to F in the tonic key of C (do-re-mi-fa).