Musica ficta (Musicology, medieval)

The music of the Middle Ages and Renaissance was often annotated under the assumption of musica ficta, which were particular raisings and lowerings of notes by the interval of a semitone, not written in the music notation. Authentic performance of such music must rely on the best available musicological scholarship to interpret the difficult and obscure rules governing when musica ficta should be introduced.

Source: How Sacred Harp music is sung – Wikipedia