on Jane Friedman site:
The “public domain” is not a place. It is a term used to describe works of authorship (books, movies, poetry, artwork) that either due to their age or their legal status under U.S. copyright law, the ability of the putative copyright owner to enforce its rights under 17 U.S.C. Section 106 is extinguished.
Boy, that’s a mouthful of lawyer-speak, right?
To explain: when a sufficiently creative idea is reduced to a tangible medium, a copyright is created.