Works whose copyright has expired or was never established, meaning anyone can use, adapt, or republish them freely.
The public domain is the collection of creative works that aren't protected by copyright, either because the copyright has expired, was forfeited, or never applied. In the US, works published before 1929 are generally in the public domain, and each January 1st another year's worth of works enters it. Government documents, works with expired copyrights, and works explicitly released by their creators all live here. You can retell, remix, adapt, or republish public domain works without asking permission or paying royalties.
Public domain works are a goldmine for writers. You can retell Greek myths, rewrite Jane Austen from a side character's perspective, or set Dracula in modern-day Tokyo without any legal issues. Some of the most successful books in recent years are creative reimaginings of public domain stories. Knowing what's available gives you a massive library of source material to build on.
Miller built a bestselling novel from Homer's Odyssey and Greek mythology, all public domain material, by giving voice to a minor character and making ancient stories feel immediate.
A prequel to Charlotte Bronte's Jane Eyre, telling the story of the 'madwoman in the attic.' Only possible because Bronte's work was in the public domain.
Took Austen's actual text and wove in zombie horror, proving that public domain works can be remixed in wildly unexpected ways and still find a huge audience.
Pick a public domain character - Dracula, Odysseus, Ebenezer Scrooge, the Phantom of the Opera, anyone published before 1929. Write a 500-word scene placing them in a modern setting with a contemporary problem. Keep their core personality but make them feel alive in today's world.