A work typically enters the Public Domain when its term of copyright expires. Determining whether or not a work is in the Public Domain can be complicated, however, as the term of copyright often differs depending on a work’s authorship, format, date of publication, and country of origin. The term "public domain" encompasses materials for which:
NOTE: Public domain is different than "publicly accessible" or "free online."
Stim, Rich. “Welcome to the Public Domain.” Copyright and Fair Use, Stanford University Libraries, fairuse.stanford.edu/overview/public-domain/welcome/.