Milligram: A unit of measurement of mass in the metric system equal to a thousandth of a gram. A gram is equal to the mass of one milliliter, one thousandth of a liter, of water at 4 degrees C. The abbreviation for milligram is mg. From Healthy Resources One Clinical Trial to Long-Term RemissionGood vs Bad Advice for Type 2 DiabetesYour EGFR-Positive NSCLC Treatment Options Featured Centers What Are the Best PsA Treatments for You?Understanding Biologics 10 Things People With Depression Wish You Knew