"Public domain" generally refers to materials that are considered "public property" and available for anyone to use for any purpose. It specifically refers to intellectual property (IP) that is not owned or controlled by anyone and can therefore be reproduced without permission or payment.
A creative work is said to be in the public domain if there are no laws which restrict its use by the public at large. It should be noted that the public domain in contrast to copyrighted works is different from the public domain in contrast to trademarks or patented works and therefore requires legal counsel. Furthermore, public domain is not the same as copyleft.
FYI: A domain name never enters public domain in the sense that copyrighted material does. It is closer in nature to a trademark, in that a failure to maintain it makes it available for others to use. If another party registers a lapsed domain name, it is no longer available to the public, as would be the case with former intellectual property which has become public domain.