The Collaborative International Dictionary of English v.0.48:

Impetus \Im"pe*tus\ ([i^]m"p[-e]*t[u^]s), n. [L., fr. impetere
   to rush upon, attack; pref. im- in + petere to fall upon,
   seek. See Petition.]
   1. A property possessed by a moving body in virtue of its
      weight and its motion; the force with which any body is
      driven or impelled; momentum.
      [1913 Webster]

   Note: Momentum is the technical term, impetus its popular
         equivalent, yet differing from it as applied commonly
         to bodies moving or moved suddenly or violently, and
         indicating the origin and intensity of the motion,
         rather than its quantity or effectiveness.
         [1913 Webster]

   2. Fig.: Impulse; incentive; stimulus; vigor; force; as, the
      President's strong recommendation provided the impetus
      needed to pass the campaign reform bill. --Buckle.
      [1913 Webster +PJC]

   3. (Gun.) The altitude through which a heavy body must fall
      to acquire a velocity equal to that with which a ball is
      discharged from a piece.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impetus
    n 1: a force that moves something along [syn: drift,
         impetus, impulsion]
    2: the act of applying force suddenly; "the impulse knocked him
       over" [syn: impulse, impulsion, impetus]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "impetus":
   acceleration, activity, animation, ardor, brio, briskness,
   catalyst, clout, compulsion, drive, driving force, elan,
   encouragement, energy, enthusiasm, getaway, glow, goad, gusto,
   impellent, impelling force, impetuosity, impulse, impulsion,
   incentive, incitation, incitement, inspiration, irresistible force,
   joie de vivre, life, liveliness, lustiness, mettle, moment,
   momentum, motivation, motive power, perkiness, pertness, pickup,
   power, propulsion, push, quickening, robustness, speedup, spirit,
   spiritedness, spur, step-up, stimulant, stimulation, stimulus,
   thrust, vivacity, warmth, zest, zestfulness