1.  mg(n = noun.quantity) milligram - one thousandth (1/1,000) gram;
is a kind of metric weight unit, weight unit
is a part of grain, metric grain
has parts: mcg, microgram

2.  mg(n = noun.substance) atomic number 12, magnesium - a light silver-white ductile bivalent metallic element; in pure form it burns with brilliant white flame; occurs naturally only in combination (as in magnesite and dolomite and carnallite and spinel and olivine);
is a kind of metal, metallic element

mg, abbr.
1 machine-gun.
2 Morris Garages (as a make of car).

mg, symb. Chem. the element manganese.

mg, abbr. milligram(s).