Share-alike
A share-alike license clause requires that any improved version of the work be shared on like terms with everyone else.
The GPL and GFDL are share-alike licenses since they require exactly that.
The Creative Commons suite of parametric licenses includes a wider range of share-alike licenses which are denoted usually as "-sa" licenses, e.g.
- CC-nc-sa which requires non-commercial use only - used for GetWiki.
In the context of Creative Commons, only, a Share Alike clause states that "If you alter, transform, or build upon this work, you may distribute the resulting work only under a license identical to this one." However, generic variations of share-alike license defines free software and open content. The term copyleft is also used to describe these terms for free software.
Open source by contrast does not require share-alike terms to be applied, and permits users to make modifications and improvements and apply a modified and more restrictive license. It is also part of the definition of open source that there can be no restrictions on field of use, which is obviously not the case for all share-alike licenses, e.g. CC-nc-sa.
