Copyleft licenses don't even need to be free! All copyleft means is that derivative works must use the same license. For example, the Creative Commons BY-NC-SA license [1] wouldn't fulfill freedom 0, since you can't use the material for commercial purposes.

(Granted, Creative Commons licenses are typically not used for software, but the point stands.)

[1] https://creativecommons.org/licenses/by-nc-sa/4.0/

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F3nd0 5 days ago | parent [-]

Is that really the case? I’m not outright disputing this, but with the term ‘copyleft’ originating from the free software movement and all, I normally take it to identify free software which protects the freedoms it grants (typically by extending the terms of its licence to derivative works).

I see that a similar mechanism is used by some non-free licences, as you have just shown, but are those really considered ‘copyleft’? Isn’t the term more properly used when said mechanism is used specifically to grant and protect the four freedoms? Both the FSF¹ and Wikipedia² seem to view the freedom aspect as an important part of copyleft, at the very least.

1. https://www.gnu.org/licenses/copyleft.en.html 2. https://en.wikipedia.org/wiki/Copyleft

	▲	jakelazaroff 5 days ago \| parent [-]
		Hm! The Wikipedia article intro explicitly lists the Creative Commons share-alike license condition in its list of "notable copyleft licenses", but later says that "any copyleft license is automatically a share-alike license but not the other way around". So at the very least I guess it's debatable :) (I'm not sure I would rely heavily on Wikipedia for this — they only use secondary sources and in practice most of their sources will be GNU-related, so the article is probably biased in that direction.)