Re:GitHub user sends notification to 400k users
by wgjordan on Sunday June 05, 2022 @12:02PM
(#31632164)
If you read the page you linked more carefully, you will see that OSI does not own a valid trademark for "Open Source", only for "Open Source Initiative".
OSI in fact tried to file for a trademark on 'Open Source' in 1999 [1], but failed because the term is 'too descriptive'.
[1] https://opensource.org/pressreleases/certified-open-source.p...
I think this is the closest you can come to a "formal definition", short of a law defining the term.