Caliper: Paper Thickness is often referred to as caliper and is measured in mils or thousandths of an inch. Not matter if you are making books or rolls of paper the caliper and number of sheets or length in feet will determine the thickness of the book or the Outside Diameter of the roll. You may find the need to measure the caliper of the paper you are using to compare it to another product or to a printer specification at some point and it would be good to do that in an accurate number.
The best way to measure would be with a Micrometer which comes in a few styles. The best way to measure paper would be to fold the paper in half, fold it in half again, measure the thickness with your gage and divide by 4. This will give you an average over a few location points and you will not capture a high or low spot.
Basis weight: At times the basis weight of the paper will go hand-in-hand with paper caliper and the basis weight identifies the paper's weight in pounds of a ream (500 sheets) in the basic size for that grade. In metric terms would be the weight in one square meter of paper or f/m2. Basis 70 would mean that 500 sheets 25" x 38" would weight 70 pounds. This is equivalent to 104 g/m2 in the metric system.
Please Note: The basic size is not the same for all paper grades (25" x 38" for coated and offset, 17" x 22" for writing papers, 20" x 26" for cover papers and there are others). Typically if you're working with similar paper your basis size will be the same.
In summary, Sensitivity, Environmental Resistance, Archivability and Caliper are the four main aspects of thermal paper that affect its grade.