Source : Webster's Revised Unabridged Dictionary (1913)

Sempiternity \Sem`pi*ter"ni*ty\, n. [L. sempiternitas.]
   Future duration without end; the relation or state of being
   sempiternal. --Sir M. Hale.

Source : WordNet®

sempiternity
     n : the property of being perpetual (seemingly ceaseless) [syn:
         {perpetuity}]