The term “public domain” encompasses those materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. No individual owns these works; rather, they are owned by the public. Anyone can use a public domain work without obtaining permission and without citing the original author, but no one can ever own it.
Copyright has expired for all works made in the United States prior to 1926. If the publication date is before January 1, 1926, then the work is in the Public Domain.
For works published after 1977, copyright will not expire until 70 years after the last surviving author dies.
Works published in the United States before 1978 immediately entered the public domain if they were generally published without a proper copyright notice. Which required the copyright symbol © (for phonorecords, the symbol ℗) word copyright of the abbreviation copr. along with the name of the holder and the date of first publication. © 1959 John Doe.
Between 1979 and 1989 works published in the United States would entered the public domain if a registration was made within 5 years of initial publication with the Copyright Office and reasonable effort was made to correct the omission on all copies distributed in the U.S. after the omission is discovered.
Works published in the United States before 1964 fall into the public domain if copyright was not renewed with the Copyright Office during the 28th year after publication. No renewal meant a loss of copyright.
For works published between 1925 and 1964, research with the Copyright Office is needed to know whether the item is in the Public Domain. For a helpful guide to researching Copyright Office records, please see this guide from Stanford University Libraries.
Sometimes, a copyright owner will choose to release their work to the Public Domain. They can do this via a CC-0 license or by placing a statement such as "This work is dedicated to the Public Domain" on their work.
It is important to verify that the person dedicating the work to the Public Domain is, in fact, the owner of the copyright for the work.
Copyright law does not protect the titles of books or movies, nor does it protect short phrases such as, “Beam me up.”
Copyright protection also doesn’t cover facts, ideas, or theories, which has important ramifications for the collection of data. While the facts of data are not subject to copyright, their organization may be. For help with questions surrounding copyright and data, contact Office of Scholarly Communication and Publishing.
(Information for this section was gleaned from "Welcome to the Public Domain" from the Stanford University Libraries.)
When using works from the Public Domain, you do not need to credit the author nor do you need to get permission, according to a 2003 ruling from the US Supreme Court.
However, it is wise to cite your sources, so crediting the original author or the source is a best practice. Be careful of copying directly from a Public Domain work, as this could qualify as plagiarism.