The term lends itself to that confusion. It’s a nebulous-sounding phrase that seems like it should represent anything that’s publicly available. After all, anything that is available on the internet is, in a way, in the public domain.
But the term public domain has a very specific meaning. In short, it is used to denote works that have no copyright protection, meaning that they can be used freely without permission from the creator or another rightsholder.
However, most content on the internet is not public domain. Though copyright does expire, it lasts (in the United States) either 95 years from publication for works of corporate authorship or the life of the author plus 70 years for works of individual authorship. In short, it’s mostly very old works that are in the public domain, currently that’s anything published before 1923.
Though there are exceptions, such as works by the federal government or works that lapsed into the public domain due to a technicality, the 1923 date rule is a relatively sound one to follow.
However, even if a work has lapsed into the public domain, that doesn’t change the rules about citation.
Whether a work is in the public domain deals only with its copyright status. That determines whether you can copy and distribute the work. It says nothing about your ethical obligations to cite and attribute it.
A good example is the works of Shakespeare. Since all of his works are in the public domain, you are completely free to post his works on your website, distribute your own books of his plays or print shirts with his sonnets.
However, if you attempt to claim Shakespeare’s work as your own, it would not just be highly foolish, but also plagiarism. This is because copyright deals with the right to copy a work and citation deals with the authorship of it. The public domain only removes your obligations to the former, not the latter.
Still, the important thing to remember is that most of the content you find on the internet is not in the public domain, even if it is publicly available, and is not free from copyright protection. However, even if the content you’re using is actually in the public domain, you still need to cite it if you are using it in your work.
The rules of plagiarism and citation apply just as much to works in the public domain as they do works still under copyright. It’s why publishers can release their own collection of Shakespeare’s works but always put his name on the cover.