(inorganic chemistry) A white powder (MgO) with a high melting point; it occurs naturally as periclase, an ore of magnesium.

(historical) A charter granted by King John to the barons at Runnymede in 1215, which is one of the bases of English constitutional tradition; a physical copy of this charter, or a later version.

(figuratively) A landmark document that sets out rights or important principles.

(physics, of a transuranic nuclide) Having filled shells of both protons and neutrons and therefore having extra stability and a longer half-life than normal.

(nautical) Any of several small bar magnets placed and adjusted within a ship's binnacle to correct for the effects of other magnetic influences in the ship.

(physics) A material, or piece of such material, which retains its magnetism even when not subjected to any external magnetic fields.

(military) A line drawn on a map or chart connecting all points at which the magnetic inclination (dip) is zero for a specified epoch.

(physics) A minor stable isotope of magnesium, ²⁶₁₂Mg, having twelve protons and fourteen neutrons; it amounts to about 11% of the element in nature

(physics) The major stable isotope of magnesium, ²⁴₁₂Mg, having twelve protons and twelve neutrons; it amounts to about 79% of the element in nature

(physics) A minor stable isotope of magnesium, ²⁵₁₂Mg, having twelve protons and thirteen neutrons; it amounts to about 10% of the element in nature