In English system of units, basis weight is the weight in pounds of a ream (500 sheets) of paper cut to a basic size. (Basic size differs from category to category of the paper. Basic size for Bond and Ledger is 20”x26”, book, offset and text paper have basic size of 25”x38”).
In metric system of units, basis weight is the weight in grams of a single sheet of area one square meter. Basis weight is also called as substance and grammage in metric system of units.