Public Domain is the status of a work that is no longer under copyright protection (the copyright has expired) or a work for which the author failed to meet statutory requirements in effect at the time the work was published. Works that are in the public domain have no restrictions on their use legally, but must still be cited appropriately to avoid plagiarism.
To determine whether a work has entered the public domain, you can consult this chart. As a general rule, only things published in the United States before 1923 are definitely in the public domain.
