As of version 7.3.33, SWI-Prolog is covered by the Simplified BSD license:

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Possible additional license requirements

Note that SWI-Prolog may be linked with libraries covered by more restrictive license and therefore the above conditions may not suffice for a particular version of SWI-Prolog or a program loaded into SWI-Prolog. Run the following predicate to find which components with additional requirements are loaded into a particular version.

?- license.

GMP

The only non-BSD component in the core system is the LGPL GNU library GMP which provides unbounded integers, rational numbers and good random numbers to SWI-Prolog. GMP can be disabled using

cmake -DUSE_GMP=OFF [more options]

Note that LGPL components do not affect the license of SWI-Prolog or applications running on top of them. However, end users of your application must be able to replace the LGPL component. See the LGPL license for details.

Libreadline

From the standard packages, only library(readline) is realised using the GPL library libreadline. This library provides command line editing on POSIX systems (MacOS, Linux, etc.). Since the migration to BSD, the default line editor is based on the BSD editline (libedit) library. End user applications generally do not require a commandline editor and are thus normally not affected. Unlike the GMP library above, the commandline editors are loaded explictly at runtime and therefore the system does not need to be recompiled to remove these components.