impetus
im.pe.tus \'im-p*t-*s\ n [L, assault, impetus, fr. impetere to attack, fr. in- + petereX to go to, seek - more at FEATHER 1a: a driving force : IMPULSE 1b: INCENTIVE, STIMULUS 2: the property possessed by a moving body in virtue of its mass and its mo tion - used of bodies moving suddenly or violently to indicate the origin and intensity of the motion