Source : Webster's Revised Unabridged Dictionary (1913)

Impetus \Im"pe*tus\, 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.

   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.

   2. Fig.: Impulse; incentive; vigor; force. --Buckle.

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

Source : WordNet®

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