A ship's metacenter (M) is the virtual intersection of two successive lines of action of the force of buoyancy when the ship heels through a very small angle. The initial position of the metacenter is used as a reference in stability calculations. The distance from a ship's center of gravity (G) to its metacenter is known as the metacentric height (GM). GM is a measure of the vessel's ability to right itself when experiencing an overturning moment. For the same vessel, a higher GM value indicates a greater initial static stability.
Source: wiktionary