Manufacturers measure paper thickness in units called "mils." One mil is equal to one one-thousandth of an inch, and thicker paper provides better dispersal of ink. The name of this measurement for paper is its "caliper."
Consumers sometimes make the mistake of assuming that paper's weight is an indicator of its thickness. Weight indicates the pounds of a 500 sheet ream of paper cut to its standard size. Different types of paper have different standard sizes. Accordingly, 80-pound cover stock paper is a different thickness than 80-pound bond paper. Caliper offers the most reliable method of comparing thickness, especially when comparing paper of different types.