The Collaborative International Dictionary of English v.0.48:

Milligram \Mil"li*gram\, Milligramme \Mil"li*gramme\, n. [F.
   milligramme; milli- milli- + gramme. See 3d Gram.]
   A measure of mass and weight, in the metric system, being the
   thousandth part of a gram, very nearly equal to the weight of
   a cubic millimeter of water, or .01543 of a grain
   avoirdupois.
   [1913 Webster] Milliliter

WordNet (r) 3.0 (2006):

milligram
    n 1: one thousandth (1/1,000) gram [syn: milligram, mg]