|A work that is in the public domain is not protected by copyright. It belongs to everyone and can be copied and shared with no restrictions. Works may be assigned to the public domain or they may enter the public domain in certain circumstances.|
Works published before 1989 without copyright notice, and without subsequent registration within 5 years are exempt from copyright protection.
Once the copyright term of a work expires, or for whatever reason was not renewed by the creator or copyright owner, it enters the public domain - meaning it belongs to everyone, without restriction.
The creator or copyright owner may decide before the expiration of copyright to dedicate the work to the public domain, giving the work to the public to use. A work that has been assigned a CC-0 Creative Commons license by its creator is in the public domain.
Works of the United States Government are excluded from copyright protections. These are born directly into the public domain, free to copy, reuse, adapt or distribute.
Please note that public availability of a work doesn’t mean it is in the public domain!