"Open source" is widely accepted to mean "licensed under terms that meet the Open Source Initiative's Open Source Definition", instead of the obvious meaning of

"Open source" is widely accepted to mean "licensed under terms that meet the Open Source Initiative's Open Source Definition", instead of the obvious meaning of "source available", only because many people put a lot of effort into correcting everybody who assumed it was plain English and not jargon. "Closed source" has not had a similar education campaign, so it still defaults to the common-sense meaning of "source not publicly available". The opposite of "open source" is "proprietary".