The Public Domain encompasses:
Public domain is different than "publicly accessible" or "free online." Read more about the Public Domain here.
CC0: "No rights reserved"
CC PD: "No known copyright"
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: