"Public domain” refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it.
There are four common ways that works arrive in the public domain:
The copyright has expired
The copyright owner failed to follow copyright renewal rules
The copyright owner deliberately places it in the public domain, known as “dedication”
Copyright law does not protect this type of work
From the Stanford Copyright and Fair Use Center’s “Welcome to the Public Domain”
Note: An important caveat regarding public domain material is that collections, new editions, and derivative works of public domain material may all be protected by copyright. With collections, an author could collect public domain works in a book or display them on a website, and the collection as a whole could be protected by copyright, even though individual works within it are not. (Boston College Libraries, “Copyright and Scholarship: Public Domain”)