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

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "puissant":
   absolute, armipotent, ascendant, authoritarian, authoritative,
   authorized, autocratic, beefy, bouncing, clothed with authority,
   cogent, commanding, competent, consequential, considerable,
   controlling, dominant, doughty, duly constituted, dynamic,
   effective, eminent, empowered, energetic, ex officio, forceful,
   forcible, forcy, full-blooded, full-strength, governing, great,
   gutsy, gutty, hale, hard, hard as nails, hardy, hearty, hefty,
   hegemonic, hegemonistic, high-potency, high-powered, high-pressure,
   high-tension, husky, imperative, important, in force, in power,
   influential, iron-hard, irresistible, leading, lusty, mighty,
   mighty in battle, momentous, monocratic, nervy, obstinate,
   official, operative, potent, powerful, preeminent, prepotent,
   prestigious, prominent, ranking, red-blooded, robust, robustious,
   rugged, ruling, senior, stalwart, steely, stout, strapping,
   striking, strong, strong as brandy, strong as strong,
   strong-willed, sturdy, substantial, superior, supreme, telling,
   totalitarian, valid, vigorous, vital, weighty