What is the definition of VOC?

There is a regulatory definition of VOC.  It is found in 40 CFR 51.100.  CFR - code of federal regulations. The definition relates to the intent and requirements of the Clean Air Act. Excerpt from “Volatile Organic Compounds” Definition per 40 CFR Part 51.100(s):
" Volatile organic compounds (VOC) means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.
(1) This includes any such organic compound other than the following, which have been determined to have negligible photochemical reactivity: methane; ethane; …. etc. "
Different VOCs have different levels of reactivity. That is, they do not react to form ozone at the same speed or do not form ozone to the same extent. Some VOCs react slowly or form less ozone; therefore, changes in their emissions have limited effects on local or regional ozone pollution episodes. The EPA lists compounds that it has determined to be negligibly reactive in its regulations as being excluded from the regulatory definition of VOCs (40 CFR 51.100(s)), i.e., these are considered to make a negligible contribution to ground-level ozone formation. Ground Level Ozone.