Public domain refers to creative works that are not protected by copyright. They belong to the public and can be used in any way without permission from the creator(s). Items can enter the public domain several ways:
It is important not to confuse public availability with public domain. The availability of a document online has nothing to do with its copyright status.
Finally, even though there isn't a legal requirement to cite public domain works, you should still do so as a matter of best practice. Not citing your sources could be considered plagiarism which can have severe professional and educational consequences.
Footnotes
1. In general:
There are lots of exceptions to these general guidelines, so make sure you do your research on each item you are interested in.
2. 17 U.S. Code § 102 "In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work."
3. Not all federal government documents are in the public domain. Work by private individuals or contractors is frequently included in government documents through the use of a license. That work still has its own copyright.
January 1st, 2022 marks the date that items published in 1926 enter the public domain. This includes Winnie the Pooh by A.A. Milne, "Someone to Watch Over Me" by George and Ira Gershwin, and The Strong Man, directed by Frank Capra.
You can find a larger list of newly public domain works on our Open Access blog.