The Collaborative International Dictionary of English v.0.48:

Periodicity \Pe`ri*o*dic"i*ty\, n.; pl. Periodicities. [Cf. F.
   p['e]riodicit['e].]
   The quality or state of being periodical, or regularly
   recurrent; as, the periodicity in the vital phenomena of
   plants. --Henfrey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

periodicity
    n 1: the quality of recurring at regular intervals [syn:
         cyclicity, periodicity]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "periodicity":
   Indian file, alternation, annual period, array, articulation, bank,
   baseball season, basketball season, beat, buzz, catena, catenation,
   chain, chain reaction, chaining, cold season, concatenation,
   concert season, connection, consecution, continuum, course, cycle,
   cyclicalness, descent, drone, dry season, endless belt,
   endless round, file, filiation, fluctuation, football season,
   frequency, frequency band, frequency spectrum, gamut, gradation,
   harmonic motion, hum, intermittence, intermittency, libration,
   line, lineage, meter, monotone, nexus, nutation, off season,
   oscillation, pendulation, pendulum, pendulum motion, period,
   periodicalness, piston motion, plenum, powder train, progression,
   pulsation, queue, rainy season, range, rank, reappearance,
   recurrence, regular wave motion, reoccurrence, resonance,
   resonance frequency, reticulation, return, rhythm, rotation, round,
   routine, row, run, scale, season, seasonableness, seasonality,
   sequence, series, single file, social season, spectrum, string,
   succession, swath, the season, thread, tier, time of year, train,
   undulation, vacillation, vibrancy, vibratility, vibration,
   wavering, windrow