The Collaborative International Dictionary of English v.0.48:

Puissant \Pu"is*sant\, a. [F., originally, a p. pr. formed fr.
   L. posse to be able: cf. L. potens powerful. See Potent.]
   Powerful; strong; mighty; forcible; as, a puissant prince or
   empire. " Puissant deeds." --Milton.
   [1913 Webster]

         Of puissant nations which the world possessed.
                                                  --Spenser.
   [1913 Webster]

         And worldlings in it are less merciful,
         And more puissant.                       --Mrs.
                                                  Browning.
   [1913 Webster]

WordNet (r) 3.0 (2006):

puissant
    adj 1: powerful