What does mg stand for?

The abbreviation"mg" stands for milligram. It is a unit of measurement of mass (which is one of the fundamental properties of matter). Other related units of measurement of mass are gram and kilogram (which is the SI unit for measurement of mass). A milligram is one-thousandth of a gram (gm), which in itself is one-thousandth of a kilogram (kg). Mathematically, 

1 mg = 0.001 gm and 1 gm =0.001 kg, 

thus, 1 mg = 0.000001 kg

That is, a milligram is one-millionth of a kilogram. 

For day to day measurement, we commonly use the kilogram. Milligrams are more commonly used for research purposes (say for making solutions) or measuring precious or expensive materials. Kilogram, gram and milligram are commonly used around the world. However, the United States uses pounds as their unit of mass.

Hope this helps.