Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The intersection of vertical lines through the center of buoyancy of a floating body when it is at equilibrium and when it is floating at an angle. The location of the metacenter is an indication of the stability of a floating body.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A midway point between a ship's centre of buoyancy when upright and its centre of buoyancy when tilted; it must be above the centre of gravity to enable a tilting ship to return to an upright position.
from the GNU version of the Collaborative International Dictionary of English
 n. 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.
from The Century Dictionary and Cyclopedia
 n. The point at which an upward thrust could be equivalent to the pressure of water upon a floating body which has received a slight rotational displacement about one of the principal axes of its section of flotation.
 n. In biology, an organism or an organ which, while one of the descendants from an archetype, itself becomes a new archetype around which new divergent or apocentric modifications are produced.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. (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
Etymologies
Examples

In other words, as long as the intersection M where the arrow Gw–B transects the metacenter plane of the vessel is above G, the ship is stable and will right itself to its original position.

NSF supercomputer centers are collaborating towards a "metacenter."

The curve may be constructed in the following manner: Having found by calculation the position of the transverse metacenter, M, for a given displacement  Figs. 1 and 2  the metacentric height, G M, is then determined either by calculations, or more correctly by experiment, by varying the position of weights of known magnitude, or by the stability indicator itself.

For a vessel with a given displacement, the metacenter and center of gravity being known, it is easy to lay off in the form of a diagram its stability or power of righting for any given angle of heel.
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