Merriam-Webster Collegiate Dictionary - impetus

 noun  Etymology: Latin, assault, ~, from impetere to attack, from in- + petere to go to, seek — more at feather  Date: 1641  1.  a.  (1) a driving force ; impulse  (2) incentive, stimulus  b. stimulation or encouragement resulting in increased activity  2. the property possessed by a moving body in virtue of its mass and its motion — used of bodies moving suddenly or violently to indicate the origin and intensity of the motion