Public domain materials are works not subject to copyright, the publicly available material that can be copied, adapted and shared, from which new knowledge and creative work can be built.
Amy Karle, image of a sculpture from the series Morphologies of Resurrection, Smithsonian Institution
While many works in the public domain are outside of copyright restrictions, some moral rights still apply. Giving credit to the original creator, for example, is an ethical acknowledgement of their work. For work digitized by galleries, libraries, archives, or museums, it is also good practice to identify the provenance of the artwork by crediting the institution and, when possible, providing a link that goes back to the institution.
Further considerations on the use of public domain materials can be found in the Public Domain Guidelines developed by the Creative Commons organization.
“The Public Domain” (https://certificates.creativecommons.org/cccertedu/chapter/2-3-the-public-domain/) by Creative Commons. CC BY 4.0.
In the U.S. copyright terms generally last for the copyright holder's lifetime, plus an additional 70 years.
In the U.S., many government publications and other works are not covered by copyright, with some exceptions.
The creator dedicates the work to the public domain A creator can dedicate their work to the public domain through a Creative Commons legal tool called CC0 ("CC Zero")
In the past there were more formal requirements to acquire or renew copyright protection. While this is no longer the case, many works have entered the public domain because a creator did not adhere to formalities.
“The Public Domain” (https://certificates.creativecommons.org/cccertedu/chapter/2-3-the-public-domain/) by Creative Commons. CC BY 4.0.