The Collaborative International Dictionary of English v.0.48:

Infrequence \In*fre"quence\, Infrequency \In*fre"quen*cy\, n.
   [L. infrequentia scantiness : cf. F. infrequence.]
   1. The state of rarely occuring; uncommonness; rareness; as,
      the infrequence of his visits.
      [1913 Webster]

   2. The state of not being frequented; solitude; isolation;
      retirement; seclusion. [R.]
      [1913 Webster]

            The solitude and infrequency of the place. --Bp.
                                                  Hall.
      [1913 Webster]

WordNet (r) 3.0 (2006):

infrequency
    n 1: noteworthy scarcity [syn: rarity, rareness,
         infrequency]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "infrequency":
   chinchiness, chintziness, exiguity, fewness, meagerness,
   miserliness, niggardliness, paucity, rarity, restrictedness,
   scantiness, scarcity, scrimpiness, skimpiness, smallness,
   sparseness, sparsity, stinginess, stringency, thinness,
   tightness