...anything that is NOT protected under US copyright law. This includes ALL works published before 1923 AND, under certain conditions, works published up to 1978. A 'work' can be anything: a book, a play, music, photographs, movies, instruction manuals, courses, reports, posters, etc...
Re-publishing and re-packaging public domain information and other creative works can make you a lot of money. The reason is fairly simple: If you find, re-package and sell information that has fallen into the public domain you do not have to pay royalties or copyright fees on that work.
The copyright law originally stated that a copyright would last for 75 years. Thus, if a book were copyrighted in 1900, the copyright would expire in 1975. If a book were published in 1920, the copyright would expire in 1995. If a book were copyrighted in 1922, the copyright would expire in 1997. But ... if a book was copyrighted in 1923 or later, then things take on a whole new slant ... courtesy of then-Congressman Sonny Bono.
Disney lobbied Congress heavily and Congressman Sonny Bono introduced legislation to extend copyright protection for an additional 20 years. This legislation passed in 1998 -- but only applied to items that were not yet in the public domain. Thus, an item copyrighted in 1992 was public domain after 75 years -- but anything copyrighted in or after 1923 was protected for 95 years.
So -- to play it perfectly safe and legal -- if the copyright date on a published work is 1922 or earlier, the item is now in the public domain and may be freely used (but not necessarily re-copyrighted and sold). If the copyright date is 1923 or later, it's best to leave it alone till at least the year 2018!