Source : Webster's Revised Unabridged Dictionary (1913)

Metacenter \Met`a*cen"ter\or -tre \-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.

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