semitone

…that differ from the standard semitones (half steps) of a tuning system or scale. In the division of the octave established by the tuning system used on the piano, equal temperament, the smallest interval (e.g., between B and C, F and F♯, A♭ and A) is the semitone, an interval…

Microtonal scales, requiring intervals smaller than the conventional half step, also appeared sporadically in the 20th century. Among microtonal structures, the most important, perhaps, have been scales calling for quarter tones (equal to half the distance of a half step).

…into 12 equal half-steps, or semitones, this method provides precisely equal intervals and a full set of chords that, although not as euphonious as those of the overtone series, are not offensive to the listener.