Works in the public domain are works that are not protected by copyright, trademark, or patent laws due to:
Every year on January 1st, some works move into the public domain because the copyright term has expired and wasn't renewed. However, this annual move to the public domain didn't occur for 20 years due to the extension of the copyright term in the Sonny Bono Copyright Term Extension Act. See this post from the Center for the Study of Public Domain at Duke Law to see what went into the public domain in 2019 after 20 year hiatus.
Need to know if a work is in the public domain? Use the public domain tools and resources listed below.
A work can be dedicated to the public domain through a Creative Commons license. Creative Commons is a nonprofit organization that provides copyright licences, public domain dedications, and copyright tools aimed at web content. Authors can choose various CC licences for their work that informs users about reuse and/or modification permissions.