In general, the term "Public Domain" means intellectual property materials that are not owned or controlled by anyone. They can be used for any purpose. Easier to understand if you say that public domain material are in direct contrast to copyrights, trademarks or patents. The creative work must have no laws to restrict useage to be in the public domain.
Public Domain laws vary from country to country. For instance, what is not restricted in Canada may be restricted in England.
Works created before the copywrite and patent laws went into force are part of the public domain.
Works of the United States Government are excluded from copyright law. So, you can use anything published by the U.S. government in your articles, books and such without concern for legality if the work was prepared by a government employee. Thus, work by U.S. private contractors may not be in the public domain.
PublicResource.org is a great place to find U.S. Government material.
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