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 in several different 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.
1. 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."
2. 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.
This work is licensed under a Creative Commons Attribution-NonCommercial 2.0 Generic License.