Diesis

In classical music from Western culture, a diesis (symbol: ♯, DY-ə-siss or enharmonic diesis, plural dieses, or "difference" The diesis is either an accidental, or a very small musical interval, usually defined as the difference between an octave and three justly tuned major thirds, equal to 128:125 or about 41.06 cents. In 12-tone equal temperament three major thirds in a row equal an octave, bu... (Wikipedia)