Source : Webster's Revised Unabridged Dictionary (1913)

Pendulum \Pen"du*lum\, n.; pl. {Pendulums}. [NL., fr. L.
   pendulus hanging, swinging. See {Pendulous}.]
   A body so suspended from a fixed point as to swing freely to
   and fro by the alternate action of gravity and momentum. It
   is used to regulate the movements of clockwork and other
   machinery.

   Note: The time of oscillation of a pendulum is independent of
         the arc of vibration, provided this arc be small.

   {Ballistic pendulum}. See under {Ballistic}.

   {Compensation pendulum}, a clock pendulum in which the effect
      of changes of temperature of the length of the rod is so
      counteracted, usually by the opposite expansion of
      differene metals, that the distance of the center of
      oscillation from the center of suspension remains
      invariable; as, the mercurial compensation pendulum, in
      which the expansion of the rod is compensated by the
      opposite expansion of mercury in a jar constituting the
      bob; the gridiron pendulum, in which compensation is
      effected by the opposite expansion of sets of rodsof
      different metals.

   {Compound pendulum}, an ordinary pendulum; -- so called, as
      being made up of different parts, and contrasted with
      simple pendulum.

   {Conical} or {Revolving}, {pendulum}, a weight connected by a
      rod with a fixed point; and revolving in a horizontal
      cyrcle about the vertical from that point.

   {Pendulum bob}, the weight at the lower end of a pendulum.

   {Pendulum level}, a plumb level. See under {Level}.

   {Pendulum wheel}, the balance of a watch.

   {Simple} or {Theoretical}, {pendulum}, an imaginary pendulum
      having no dimensions except length, and no weight except
      at the center of oscillation; in other words, a material
      point suspended by an ideal line.

Ballistic \Bal*lis"tic\, a.
   1. Of or pertaining to the ballista, or to the art of hurling
      stones or missile weapons by means of an engine.

   2. Pertaining to projection, or to a projectile.

   {Ballistic pendulum}, an instrument consisting of a mass of
      wood or other material suspended as a pendulum, for
      measuring the force and velocity of projectiles by means
      of the arc through which their impact impels it.

Source : WordNet®

ballistic pendulum
     n : a physical pendulum consisting of a large mass suspended
         from a rod; its displacement is used to measure the
         velocity of a projectile