[sanddrag]

It would be the bolt's number or major diameter. Bolts below 1/4" are measured by numbers (up to the number 12). Each number corresponds to a diameter (by reference chart, probably created by ANSI). Bolts 1/4" and over are usually specified by fractional (like 7/16"), whole (1"), or mixed (1 1/2") number. It should be noted that there is such thing as a #0 screw, a #00 screw, and I believe even a #000 screw (not positive on that last one though).

Also, a thread designation is typically followed by thread class. There are six thread classes 1A, 2A, 3A which apply to external threads and 1B, 2B, 3B which apply to internal threads. 1 is a loose fitting thread, for ease of assembly or use in a dirty environment. 2 is the most common thread fit. A slight amount of dirt will not jam it up but too much will. 3 is a close tolerance thread fit meant for precision and clean environments.