Public-domain software is software that has been placed in the public domain, in other words, software for which there is absolutely no ownership such as copyright, trademark, or patent. Software in the public domain can be modified, distributed, or sold even without any attribution by anyone; this is unlike the common case of software under exclusive copyright, where licenses grant limited usage rights.
Under the Berne Convention, which most countries have signed, an author automatically obtains the exclusive copyright to anything they have written, and local law may similarly grant copyright, patent, or trademark rights by default. The Convention also covers programs, and they are therefore automatically subject to copyright. If a program is to be placed in the public domain, the author must explicitly disclaim the copyright and other rights on it in some way, e.g. by a waiver statement. In some jurisdictions, some rights (in particular moral rights) cannot be disclaimed: for instance, civil tradition-based German law's "Urheberrecht" differs from Anglo-Saxon common law tradition's "copyright" concept.
