No (or very few) Restrictions
Public Domain: Works in the public domain may be used without copyright infringement. This distinction is not as clear cut as it may sound; it may not be a copyright infringement but it could be a patent or trademark violation. This guide is intended to illustrate public domain as it applies to copyright only.
What does "in the public domain" mean?
| U.S. Government Works | From U.S. copyright law: Copyright protection “is not available for any work of the United States Government.” This could include federal judicial decisions, statutes, speeches of federal government officials, press releases, census reports, etc. For example, "The Visible Body" is produced by the NLM and is an openly and freely accessible series of images about the human body. |
| Copyright expired | In the United States, the length of the term of copyright is life of the author plus an additional 70 years. |
| Author chose it | CC0 Public Domain Dedication allows anyone to waive their copyright and place a work directly into the global public domain. CC0 stands for Creative Commons 0. There are several levels of licensing at Creative Commons; only 0 level has no restrictions whatsoever. |
| Not subject to copyright protection in the first place | There are some types of works which don’t get copyright, including:
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Show Me
The NLM (National Library of Medicine) coordinates The Visible Human Project. Many of its resources are government-created and therefore are in the public domain.
It is important to note that it is government-created works that are, by nature, in the public domain. A government-sponsored webpage or program such as this may very well include works that do fall under copyright.
The Copyright link is in the bottom left corner: 
Clicking it brings you to this notice: 
