The definitions of open source are actually quite technical. They are not vague like you suggest.

It's unavoidable that people will be confused about terms outside their area of expertise. Since you mentioned gamers: I shudder at the vast multitude of confusing terms the various gamer communities use, like for example in WoW.

However, I think you're overstating your case: I suspect few gamers, if any, will confuse leaked source code or code which you're not allowed to use in any way as "open source". I don't expect them to undestand every nuance, but surely even they understand source code you're not allowed to run or modify is not "open" in any meaningful interpretation of the term :)