Basis Weight

The weight of a ream of paper in the basic size for that grade (e.g.
500 sheets of 20 Bond paper in its basic size, 17x22 in. [4
standard sheets], would weigh 20 lb.)

Sometimes the same paper is assigned two basis weights, e.g. 24/60,
meaning the same paper is sold in two different basis sizes. 24 lb.
bond (basis size 17"x22") is approximately the same weight as 60 lb.
text or offset (basis size 25"x38").

You should note that paper can have different densities and thus
different thickness are possible for the same weight – such as
cover stock!