Any publication issued at government expense or published under the authority of a governmental body.
Included are official papers that record the actions or deliberations of government (Congressional Record), informational publications (statistical compilations of the Bureau of the Census), and reports of research done under government contract.
There are many ways to utilize government documents into your research paper. They are great to use as a primary source and cover most topics.
The United States Government, through the Government Printing Office (GPO) and its predecessor agencies has provided open access to much U.S. information since 1790.