Source : Webster's Revised Unabridged Dictionary (1913)

Milligram \Mil"li*gram\, Milligramme \Mil"li*gramme\, n. [F.
   milligramme; milli- milli- + gramme. See 3d {Gram}.]
   A measure of weight, in the metric system, being the
   thousandth part of a gram, equal to the weight of a cubic
   millimeter of water, or .01543 of a grain avoirdupois.