Creative Commons Zero
Creative Commons Zero (abbreviated CC0 or CC-0) is a waiver that allows an author to give up all their rights, including copyrights and moral rights, placing their work into public domain. It is often inaccurately labeled a license (likely out of similarity to licenses such as CC-BY-SA) ‒ while it does contain an unconditional, irrevocable, non exclusive, royalty free fallback license in case the waiver is ineffective under a specific law, the main purpose is to give up all rights, as opposed to granting some rights to others. It can be though of as no rights reserved option.
Unlike other CC licenses, CC0 is suitable not only for media, but also for the source code, and is compatible with many free software licenses including the GPL. For public domain software it is the recommended option by the Free Software Foundation.