When I am presenting my research today, I find I have to clarify the Public Domain for myself and others in brief and clear sentences. Here it is.
Basically, Public Domain means the Domain that is out of the Private Right. It has many definitions such as the strict legal definition, broader definition and the multi-discipline ones.
The Strict one, and it is a traditional one in the context of copyright law, is: A domain that any works falling Into it can be freely used by the public, and this domain should be able to logically derived from the regulation of copyright law.
The broader one is: A domain that contains anything that should not be controlled by the private owners, not just literary and music works, but also the facts, ideas, public owned lands and other properties and non-properties.
The multi-discipline ones are relevant to many aspects. For example: the public domain in the context of Jürgen Habermas' political philosophy, the public goods in the sense of institutional economics, and others.
I had reviewed most of the relevant conceptions on this issue before and proposed my own definition either from the law or from the fact of the contemporary digitalized world. It is a combination of the de facto public domain and the de jure public domain. When I am doing legal interpretation, I will focus on the de jure public domain (It is mostly the strict legal one, but at the same time, the theories of the other definitions are included). When I am analyzing the reasons and obstacles of the legal reform, I will observe the de facto public domain. Then compare them, then look for the reasons why the de jure and the de facto has distinctions from each other.
\My version of Public Domain is of cource standing on the Giant's shoulders, and at the same time focusing on my research theme of Copyright Reform and Social Development ... not my dissertation, but a longer and bigger plan of my research in the future.