Basis weight
Also: basis weight
The mass of a standard number of sheets at a reference size, used in North America alongside grammage (g/m²).
Basis weight is the North American convention for measuring paper weight. It reports the mass in pounds of 500 sheets (a ream) cut to the grade's reference size. The reference size changes by grade family, which is why a 20-lb copy paper and a 20-lb offset are not the same gsm.
Copy paper reference size is 17 x 22 inches, so 20-lb bond equals about 75 g/m². Cover paper reference is 20 x 26 inches, so 65-lb cover equals about 176 g/m². Most of the rest of the world uses grammage (g/m²) directly, and industry data increasingly lists both.