(from Greek dia pasōn chordōn: “through all the strings”), in medieval music, the interval, or distance between notes, encompassing all degrees of the scale—i.e., the octave. In French, diapason indicates the range of a voice and is also the word for a tuning fork and for pitch.

On the organ, the open and stopped diapason are two basic stops, or ranks of pipes sounding a given tone colour. Open diapason pipes are also known as principals.