Sovereign nation.

1-Define: What is a sovereign nation.
2-How many sovereign nations are recognized as members by the United Nations?
3-How many sovereign nations are recognized by the US State Department?
4-Are the number of sovereign nations recognized by each (UN and US State Dept.) the same?
5-Are the criteria used the same? If not, what are the criteria and how do they differ?
6-It is not necessary to post the entire list of sovereign nations.