A musical interval at which a scale repeats its intervallic structure. For tunings which recognize a period, the period is also a generator.
The most typical example of a period is the octave, whose two frequencies form exactly or approximately the 2:1 ratio. When the octave is recognized as the period, notes which are an octave apart have the same name.
The "interval of periodicity" = "periodic interval" = "period" is that generator of a regular temperament (whether linear [1-dimensional], planar [2-dimensional], or n-dimensional) which generates the interval of equivalence all by itself. This means that the period is either equal to the interval of equivalence or fits into the interval of equivalence a whole number of times.