A public domain work is a creative work that is not protected by copyright because the copyright term is expired or copyright never existed. The work may be freely used by everyone.
Works fall into the public domain for three main reasons:
As a general rule, most works enter the public domain because of old age. This includes any work published in the United States before 1923. Another large block of works are in the public domain because they were published before 1964 and copyright was not renewed. (Renewal was a requirement for works published before 1978.) A smaller group of works fell into the public domain because they were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).
Use the Copyright Slider Tool to determine if a work is still protected by copyright.