The distance between two frequencies, often expressed in semitones (or cents). One semitone is equal to 100 cents. The twelve-tone equal temperament scale divides an octave into 12 semitones.
A theoretical model of an equidistant heptatonic scale, where all the intervals of the seven-note scale were perfectly equal, would result in an interval of 1.714 semitones each.
Knowing the exact scale used will allow you to easily reproduce the tonal "color" characteristics of a particular scale when creating an instrument or transferring it to another key.