The Collaborative International Dictionary of English v.0.48:

Metacenter \Met`a*cen"ter\or Metacentre \Met`a*cen"tre\, n.
   [Pref. meta- + center.] (Hydrostatics)
   The point of intersection of a vertical line through the
   center of gravity of the fluid displaced by a floating body
   which is tipped through a small angle from its position of
   equilibrium, and the inclined line which was vertical through
   the center of gravity of the body when in equilibrium.
   [1913 Webster]

   Note: When the metacenter is above the center of gravity, the
         position of the body is stable; when below it,
         unstable.
         [1913 Webster]

WordNet (r) 3.0 (2006):

metacenter
    n 1: (shipbuilding) the point of intersection between two
         vertical lines, one line through the center of buoyancy of
         the hull of a ship in equilibrium and the other line
         through the center of buoyancy of the hull when the ship is
         inclined to one side; the distance of this intersection
         above the center of gravity is an indication of the
         stability of the ship [syn: metacenter, metacentre]