Where is the Official list of Windows Environment Variables?

While not official the best functional reference I've seen, that has been reliable and kept up to date for many years, is SS64 http://ss64.com/nt/syntax-variables.html. It includes undocumented variables and OS version specific info.

Site? Maybe not? Try this link. The list even breaks it down to what OS can use it. The variable will look a little different. That is only because it is missing the percent signs that are at the beginning and end.

Still not happy? Try this opening Command Prompt and type set and hit enter. It will list all the variables. As with the site listed above, the variable here will also look a little different.