n. A large wine-bottle, usually twice the size of the ordinary bottle used for the same kind of wine.n. The quantity of wine contained in such a bottle: as, a magnum of port.n. Pl. magna (-nä). In anatomy, the largest bone of the human carpus, in the distal row, between the trapezoid and the unciform, in special relation with the head of the middle metacarpal bone: more fully called os magnum.