Public Domain

From Wikipedia

Public domain books are those to which no person or other legal entity can establish or maintain proprietary interests within a particular legal jurisdiction. This body of information and creativity is considered to be part of a common cultural and intellectual heritage, which, in general, anyone may use or exploit, whether for commercial or non-commercial purposes. Only about 15 percent of all books are in the public domain, and 10 percent of all books that are still in print. (Source: New York Times; May 14, 2006; Scan This Book!)

If an item (“work”) is not in the public domain, this may be the result of a proprietary interest such as a copyright, patent, or other sui generis right. The extent to which members of the public may use or exploit the work is limited to the extent of the proprietary interests in the relevant legal jurisdiction. However, when the copyright, patent or other proprietary restrictions expire, the work enters the public domain and may be used by anyone for any purpose.

Learn more about expiration laws and more on the Wikipedia Public Domain article

Also read:

Public Domain Explained by David Welch