gram (Gm, g), n the basic unit of mass of the metric system. Equivalent to 15.432 gr. 1. the basic unit of mass (weight) of the metric system, being the equivalent of 15.432 grains; symbol g, sometimes abbreviated gm. See also Table 4.2 and si units. 2. see gram's stain. 3. see chickpea. gram atomic weight. the same numerically as the atomic weight of an element but expressed in grams.