Are Copyleft and Share-Alike synonyms?

Question

Copyleft and Share-Alike are two very similar types of licences. Here is how Wikipedia defines them:

Copyleft (a play of the word copyright) is the practice of offering people the right to freely distribute copies and modified versions of a work with the stipulation that the same rights be preserved in derivative works down the line. (Source)

Share-alike is a copyright licensing term, originally used by the Creative Commons project, to describe works or licences that require copies or adaptations of the work to be released under the same or similar licence as the original. Copyleft licences are free content or free software licences with a share-alike condition. (Source)

Are there any meaningful distinctions between these concepts, or is the only real difference their heritage and most common uses (software vs media)?

Answer 1

The terms "copyleft" and "ShareAlike" both refer to reciprocal licensing, in which downstream users receive the same rights to your changes that you received for the original work. But copyleft is a stricter concept than ShareAlike, adding a requirement that derivatives be made available in a form that gives downstream users the technical ability to make uses of a work that the license permits.

• Copyleft licenses designed for computer programs, such as the GNU General Public License, require licensees to publish the derivative's "source code", which GPL versions 2 and 3 define as "the preferred form of the work for making modifications to it."
• The GNU Free Documentation License, a copyleft license for instructional works, requires licensees to publish something similar called a "Transparent copy", which it defines as "a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters."

Answer 2

They are synonyms, to the extent where copyleft is considered to be a large subcategory of ShareAlike. There isn't much distinction. Creative Commons is the one that coined the term, ShareAlike.

Both practices are seen throughout: copyleft is present in many things, from software to literature, and the same is for ShareAlike. To understand why one practice is mostly applied in a field, rather than the other, would be to look at the nature of the licenses, and the organizations that foster them. Creative Commons is used much more for creative works, and as they coined the term ShareAlike, it's understandable that this would be seen more in creative works. Likewise can be said for Copyleft, where it was first seen in the GNU public license. Therefore, it was seen more often in Information Technology sectors.

There isn't really much difference, and their varying applications are not a result of the ideologies themselves.