The Public Domain encompasses:
Public domain is different than "publicly accessible" or "free online." Read more about the Public Domain here.
Most content found on .gov websites by the US Federal Government is free of any copyright restrictions and in the public domain. However, look closely at .gov content--sometimes the materials have been contracted out or developed in partnership with a company or organization, and in these cases, a traditional all rights reserved copyright may be present.
Here are some examples of US Federal Databases: