WordNet (r) 3.0 (2006):

will power
    n 1: the trait of resolutely controlling your own behavior [syn:
         self-control, self-possession, possession,
         willpower, will power, self-command, self-will]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "will power":
   animus, appetence, appetency, appetite, choice, command, conation,
   conatus, decision, desire, determination, discretion, disposition,
   fancy, fixity of purpose, fortitude, free choice, free will,
   inclination, intention, iron will, liking, lust, mind,
   moral courage, moral fiber, objective, passion, pleasure, power,
   resolution, sexual desire, strength, strength of mind,
   strength of purpose, velleity, volition, will, will of iron,
   wish