The Doom Source License (or DSL) is the original source code license under which the Doom source code was released in late 1997. The Doom source was later re-released under the GNU GPL. As the license cannot be revoked, the Doom source is essentially dual licensed under the DSL and the GPL. Confusingly, source ports which use the DSL are not Open Source, even if the source code is still available; this is because the DSL does not meet the requirements for what is considered "open" under the Open Source Definition. Similarly, DSL source ports are also non-Free Software.