in·fre·quen·cy /-kwən(t)si/
  稀少;罕有
  In·fre·quence In·fre·quen·cy n.
  1. The state of rarely occuring; uncommonness; rareness; as, the infrequence of his visits.
  2. The state of not being frequented; solitude; isolation; retirement; seclusion. [R.]
     The solitude and infrequency of the place.   --Bp. Hall.
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  infrequency
       n : noteworthy scarcity [syn: rarity, rareness]