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]