impetus

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 ]

☞ 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 ]