Source : Webster's Revised Unabridged Dictionary (1913)

Impermanence \Im*per"ma*nence\, Impermanency \Im*per"ma*nen*cy\,
   n.
   lack of permanence.

Source : WordNet®

impermanency
     n : the property of not existing for indefinitely long durations
         [syn: {impermanence}] [ant: {permanence}]