What is the public domain?

The “public domain” includes all inventions, creations, etc. which are not protected by any intellectual property rights.  Such information is not owned by anybody, which means anyone is entitled to use or copy it.

So the public domain includes information that has never been protected by any intellectual property rights.  For example, the title of a book or movie is not something which can be protected by copyright, so you could write a book called “The DaVinci Code” without infringing Dan Brown’s copyright in the book, assuming that your book has nothing in common other than the title.

The public domain also includes information that was protected at one time but no longer has any such protection.  For example, when a patent expires, anyone can make or use the invention that was previously protected by that patent.  So even though Thomas Edison patented the light bulb in the 1880’s, that patent has long since expired and anyone can now make a basic light bulb.