CC BY-SA 4.0 is the copyleft Creative Commons licence. Anyone may share, remix, adapt, and build on the material for any purpose, including commercial, provided they give appropriate credit and license their adaptations under the same licence or a CC-approved compatible licence.
It is the right choice for content that should stay in the commons even as it is remixed. Wikipedia uses CC BY-SA 4.0 in combination with the GNU Free Documentation License.
How to apply
Attach the licence to the work and include an attribution line that also declares the share-alike requirement:
"Work title" by Author is licensed under CC BY-SA 4.0.
https://creativecommons.org/licenses/by-sa/4.0/
Full license text
Attribution-ShareAlike 4.0 International
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Creative Commons Corporation ("Creative Commons") is not a law firm and
does not provide legal services or legal advice. Distribution of
Creative Commons public licenses does not create a lawyer-client or
other relationship.
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Creative Commons Attribution-ShareAlike 4.0 International Public License
By exercising the Licensed Rights (defined below), You accept and agree
to be bound by the terms and conditions of this Creative Commons
Attribution-ShareAlike 4.0 International Public License ("Public
License").
Section 1 -- Definitions.
Section 2 -- Scope.
Section 3 -- License Conditions.
a. Attribution (see CC-BY-4.0).
b. ShareAlike.
In addition to the conditions in Section 3(a), if You Share
Adapted Material You produce, the following conditions also
apply.
1. The Adapter's License You apply must be a Creative Commons
license with the same License Elements, this version or later,
or a BY-SA Compatible License.
2. You must include the text of, or the URI or hyperlink to, the
Adapter's License You apply.
3. You may not offer or impose any additional or different terms
or conditions on, or apply any Effective Technological
Measures to, Adapted Material that restrict exercise of the
rights granted under the Adapter's License You apply.
Section 4 -- Sui Generis Database Rights.
Section 5 -- Disclaimer of Warranties and Limitation of Liability.
Section 6 -- Term and Termination.
Section 7 -- Other Terms and Conditions.
Section 8 -- Interpretation.
… this text is abbreviated.
For the complete license text, see
https://creativecommons.org/licenses/by-sa/4.0/legalcode
Raw text also available at /license-texts/CC-BY-SA-4.0.txt — handy for curl.