The Collaborative International Dictionary of English v.0.48:

Impermanence \Im*per"ma*nence\, Impermanency \Im*per"ma*nen*cy\,
   n.
   Lack of permanence.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impermanence
    n 1: the property of not existing for indefinitely long
         durations [syn: impermanence, impermanency] [ant:
         permanence, permanency]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "impermanence":
   adaptability, adjustability, alterability, caducity, changeability,
   changeableness, changefulness, corruptibility, death, ephemerality,
   ephemeralness, evanescence, finitude, fleetingness, flexibility,
   fluidity, fugacity, impermanency, instability, malleability,
   mobility, modifiability, momentariness, mortality, movability,
   mutability, nonuniformity, perishability, permutability,
   plasticity, resilience, rubberiness, suppleness, transience,
   transiency, transientness, transitoriness, volatility