To avoid confusion, some people use the words "libre" and "gratis" to avoid the ambiguity of the English word "free".

To avoid confusion, some people use the words "libre" and "gratis" to avoid the ambiguity of the English word "free". However, these alternative terms are still used mostly within the free software movement and are only slowly spreading to the outside world.
Others advocate the term open source software. However, according to the FSF, the Open Source movement is philosophically distinct from the free software movement. See the discussion below (Comparison with Open Source software).
There are several variations on free software in the FSF sense, for example:
The freedoms defined by the FSF are protected through copyleft licenses, the most prominent of which is the GNU General Public License. The author retains copyright, and permits redistribution and modification under terms designed to ensure that all modified versions of the software remain under copyleft terms. Public domain software, in which the author has abandoned the copyright. Public-domain software, since it is not protected by copyright at all, may be freely incorporated into closed, proprietary works as well as free ones. BSD-style licenses, so called because they are applied to much of the software distributed with the BSD operating systems. The author under such licenses retains copyright protection solely to disclaim warranty and to require proper attribution of modified works, but permits redistribution and modification, even in proprietary works. Note that the original copyright owner of copyleft-licensed software can also make a modified version under their original copyright, and sell it under any license they like, in addition to distributing the original version as free software. This technique has been used as a business model by a number of free software companies; this does not restrict any of the rights granted to the users of the copyleft version