The Collaborative International Dictionary of English v.0.48:

Puissance \Pu"is*sance\, n. [F., fr. puissant. See Puissant,
   and cf. Potency, Potance, Potence.]
   Power; strength; might; force; potency. " Youths of
   puissance." --Tennyson.
   [1913 Webster]

         The power and puissance of the king.     --Shak.
   [1913 Webster]

   Note: In Spenser, Shakespeare, and Milton, puissance and
         puissant are usually dissyllables.
         [1913 Webster]

WordNet (r) 3.0 (2006):

puissance
    n 1: power to influence or coerce; "the puissance of the labor
         vote"