It's a tricky problem, because I'm not sure any word clearly encompasses the idea that "the user has the right to modify it and share their modifications". Open-source was one attempt, but has the other confusion that it's only about availability of source, not freedom to use/modify it. And RMS dislikes the OSI for other reasons, some of which are that they de-emphasize this user-centric freedom and instead focus more on things like why open-source software improves software reliability.

"Creative commons" is a more successful attempt, but has a strong art/music/literature connotation (which is where it's caught on more, with associated phrases like "right to remix" that get at the same idea, but in a very media-focused way).