The Collaborative International Dictionary of English v.0.48:

Percussion \Per*cus"sion\ (p[~e]r*k[u^]sh"[u^]n), n. [L.
   percussio: cf. F. percussion. See Percuss.]
   1. The act of percussing, or striking one body against
      another; forcible collision, esp. such as gives a sound or
      report. --Sir I. Newton.
      [1913 Webster]

   2. Hence: The effect of violent collision; vibratory shock;
      impression of sound on the ear.
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            The thunderlike percussion of thy sounds. --Shak.
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   3. (Med.) The act of tapping or striking the surface of the
      body in order to learn the condition of the parts beneath
      by the sound emitted or the sensation imparted to the
      fingers. Percussion is said to be immediate if the blow is
      directly upon the body; if some intervening substance, as
      a pleximeter, is, used, it is called mediate.
      [1913 Webster]

   Center of percussion. See under Center.

   Percussion bullet, a bullet containing a substance which is
      exploded by percussion; an explosive bullet.

   Percussion cap, a small copper cap or cup, containing
      fulminating powder, and used with a percussion lock to
      explode gunpowder.

   Percussion fuze. See under Fuze.

   Percussion lock, the lock of a gun that is fired by
      percussion upon fulminating powder.

   Percussion match, a match which ignites by percussion.

   Percussion powder, powder so composed as to ignite by
      slight percussion; fulminating powder.

   Percussion sieve, Percussion table, a machine for sorting
      ores by agitation in running water.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

center \cen"ter\ (s[e^]n"t[~e]r), n. [F. centre, fr. L. centrum,
   fr. Gr. ke`ntron any sharp point, the point round which a
   circle is described, fr. kentei^n to prick, goad.]
   1. A point equally distant from the extremities of a line,
      figure, or body, or from all parts of the circumference of
      a circle; the middle point or place.
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   2. The middle or central portion of anything.
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   3. A principal or important point of concentration; the
      nucleus around which things are gathered or to which they
      tend; an object of attention, action, or force; as, a
      center of attaction.
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   4. The earth. [Obs.] --Shak.
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   5. Those members of a legislative assembly (as in France) who
      support the existing government. They sit in the middle of
      the legislative chamber, opposite the presiding officer,
      between the conservatives or monarchists, who sit on the
      right of the speaker, and the radicals or advanced
      republicans who occupy the seats on his left, See Right,
      and Left.
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   6. (Arch.) A temporary structure upon which the materials of
      a vault or arch are supported in position until the work
      becomes self-supporting.
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   7. (Mech.)
      (a) One of the two conical steel pins, in a lathe, etc.,
          upon which the work is held, and about which it
          revolves.
      (b) A conical recess, or indentation, in the end of a
          shaft or other work, to receive the point of a center,
          on which the work can turn, as in a lathe.
          [1913 Webster]

   Note: In a lathe the

   live center is in the spindle of the head stock; the

   dead center is on the tail stock.

   Planer centers are stocks carrying centers, when the object
      to be planed must be turned on its axis.
      [1913 Webster]

   Center of an army, the body or troops occupying the place
      in the line between the wings.

   Center of a curve or Center of a surface (Geom.)
      (a) A point such that every line drawn through the point
          and terminated by the curve or surface is bisected at
          the point.
      (b) The fixed point of reference in polar coordinates. See
          Coordinates.

   Center of curvature of a curve (Geom.), the center of that
      circle which has at any given point of the curve closer
      contact with the curve than has any other circle whatever.
      See Circle.

   Center of a fleet, the division or column between the van
      and rear, or between the weather division and the lee.

   Center of gravity (Mech.), that point of a body about which
      all its parts can be balanced, or which being supported,
      the whole body will remain at rest, though acted upon by
      gravity.

   Center of gyration (Mech.), that point in a rotating body
      at which the whole mass might be concentrated
      (theoretically) without altering the resistance of the
      intertia of the body to angular acceleration or
      retardation.

   Center of inertia (Mech.), the center of gravity of a body
      or system of bodies.

   Center of motion, the point which remains at rest, while
      all the other parts of a body move round it.

   Center of oscillation, the point at which, if the whole
      matter of a suspended body were collected, the time of
      oscillation would be the same as it is in the actual form
      and state of the body.

   Center of percussion, that point in a body moving about a
      fixed axis at which it may strike an obstacle without
      communicating a shock to the axis.

   Center of pressure (Hydros.), that point in a surface
      pressed by a fluid, at which, if a force equal to the
      whole pressure and in the same line be applied in a
      contrary direction, it will balance or counteract the
      whole pressure of the fluid.
      [1913 Webster] Center