Public domain is a designation for content that is not protected by any copyright law or other restriction and may be freely copied, shared, altered and republished by anyone. The designation means, essentially, that the content belongs to the community at large. Copyright restrictions vary among types of content and different countries.
Content in the public domain is usually there because either the copyright or restriction has expired or because it was never protected, often because the content owner has purposely placed it in the public domain. Early silent films, for example, are usually in the public domain because their copyrights have expired. Musicians sometimes release works directly into the public domain so that it may be freely accessed.
Music, films, literature, images, software, patents and mathematical formulas are among the wide variety of intellectual property that may be in the public domain.