On their website, packaging experts at the Paperboard Packaging Council offer their Industry Glossary. Definitions include:
basis weight: the specification of boxboard as density of weight per unit area. In the US it is measured as “pounds per thousand square feet,” and in Europe, as “grams per square meter”(gsm). It ranges from a light weight of 60 pounds per thousand square feet to as heavy as 200 pounds per thousand square feet in a single ply; however, any one machine is ordinarily not capable of making this complete range.