The public domain is the wealth of cultural materials that can be freely used, absent any form of copyright protections. The public domain is essential to a creative, productive society as it creates a place where ideas can be exchanged, developed, used, and reused without any legal or monetary barriers. Even in its original creation of copyright protections, the constitution ensured that these protections would eventually expire and all ideas would enter the public domain.
There are several ways works can enter the public domain:
Copyright protection has expired
Copyright is a temporary right granted to authors. Once this temporary protection has come to its end, all legal restrictions cease to exist, subject in some countries to the author's perpetual moral rights. For more information on when copyrights expire in the U.S. visit this chart from Cornell University.
Work is not covered by copyright
Some works are not protected by copyright because they fail the test of originality, or are excluded from protection. These include data, facts, ideas, procedures, processes, systems, methods of operation, concepts, principles, or discoveries, regardless of the form in which they are described, explained, illustrated, or embodied in a work, as well as laws and judicial and administrative decisions. Works created by government bodies are not protected by copyright.
Work is voluntarily shared by the creator
Creators can remove use restrictions from their works by either freely licensing them, or by using other legal tools to allow others to use their works without restrictions, or by dedicating them to the Public Domain. Things like the creative commons license allows works to be freely used and distributed, though this is not the same thing as being in the public domain, it generates many of the same benefits.
Is a reprint of a public domain work protected by copyright?
Not generally, unless creative modifications are made. These modifications could include the addition or removal of any editorial material and artistic contributions, such as illustrations or a new arrangement of the public domain material. For these reasons, a new edition or a derivative work an existing public domain work may be not be able to be reproduced without the permission of the copyright owner.
Is a translation of a public domain work protected by copyright?
Yes. A translation is a derivative work of the original and is protected by copyright. The permission of the copyright owner is needed to translate the owner’s work into another language.