A gram is a unit of measure part of the metric system. It measures mass, or how much matter is in something. The symbol g often means the gram. One gram used to be defined as the mass of one cubic centimeter of pure water at 3.98 degrees Celsius. Now one gram is defined as 1/1000 kilogram. A milligram is 1/1000 of a gram.
Gram Facts for Kids. Kiddle Encyclopedia.