The Collaborative International Dictionary of English v.0.48:

Vis \Vis\, n.
   1. Force; power.
      [1913 Webster]

   2. (Law)
      (a) Physical force.
      (b) Moral power.
          [1913 Webster]

   Principle of vis viva (Mech.), the principle that the
      difference between the aggregate work of the accelerating
      forces of a system and that of the retarding forces is
      equal to one half the vis viva accumulated or lost in the
      system while the work is being done.

   Vis impressa [L.] (Mech.), force exerted, as in moving a
      body, or changing the direction of its motion; impressed
      force.

   Vis inertiae. [L.]
      (a) The resistance of matter, as when a body at rest is
          set in motion, or a body in motion is brought to rest,
          or has its motion changed, either in direction or in
          velocity.
      (b) Inertness; inactivity.

   Note: Vis intertiae and inertia are not strictly synonymous.
         The former implies the resistance itself which is
         given, while the latter implies merely the property by
         which it is given.

   Vis mortua [L.] (Mech.), dead force; force doing no active
      work, but only producing pressure.

   Vis vitae, or Vis vitalis [L.] (Physiol.), vital force.


   Vis viva [L.] (Mech.), living force; the force of a body
      moving against resistance, or doing work, in distinction
      from vis mortua, or dead force; the kinetic energy of a
      moving body; the capacity of a moving body to do work by
      reason of its being in motion. See Kinetic energy, in
      the Note under Energy. The term vis viva is not usually
      understood to include that part of the kinetic energy of
      the body which is due to the vibrations of its molecules.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

VIS IMPRESSA. Immediate force; original force. This phrase is applied to
cases of trespass when a question arises whether an injury has been caused
by a direct force, or one which is indirect. When the original force, or vis
impressa, had ceased to act before the injury commenced, then there is no
force, the effect is mediate, and the proper remedy is trespass on the case.
     2. When the injury is the immediate consequence of the force or vis
proxima, trespass vi et armis lies. 3 Bouv. Inst. n. 3483; 4 Bouv. Inst. n.
3583.