As per wikipedia:
"Thread count or threads per inch (TPI)[SUP]
[6][/SUP] is a measure of the coarseness or fineness of fabric. It is measured by counting the number of threads contained in one square inch of fabric or one square centimeter, including both the length (
warp) and width (
weft) threads. The thread count is the number of threads counted along two sides (up and across) of the square inch, added together. It is used especially in regard to cotton
linens such as
bed sheets, and has been known to be used in the classification of towels."
There is a thread counter that you can buy specially for that purpose. It's a magnifying
glass with a 1 inch square. Used it when I was working in textiles.