What is State Paper?

Legal Definition
A State Paper is a document or file kept by a government to record discussions, options and decisions by government officials, departments and civil servants. Some states follow a thirty year rule whereby state papers on an issue may be released to academic scrutiny thirty years after an original discussion or decision.

State papers are often kept in a country's National Archives, State Paper Office or Public Record Office. All files are numbered using an alphanumeric code which academics may use as a reference in footnotes of books.

Some state papers are embargoed for reasons of national security or other sensitive reasons.
-- Wikipedia
Legal Definition
A document prepared by, or relating to, the political department of the government of a state or nation, and concerning or affecting the administration of its government or its political or international relations. Also, a newspaper, designated by public authority, as the organ for the publication of public statutes, resolutions, notices, and advertisements.
-- Black's Law Dictionary