Source : Webster's Revised Unabridged Dictionary (1913)

Transience \Tran"sience\, Transiency \Tran"sien*cy\, n.
   The quality of being transient; transientness.

Source : WordNet®

transience
     n 1: an impermanence that suggests the inevitability of ending or
          dying [syn: {transiency}, {transitoriness}]
     2: the attribute of being brief or fleeting [syn: {brevity}, {briefness}]