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The Collaborative International Dictionary of English v.0.48 (gcide)

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]