The US stands for United States. So, you will often see the American leader referred to as "the president of the United States." He is sometimes referred as the POTUS (President of the United States).
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Thomas Jefferson was the president when the first US census was made.
This depends on the country. See related questions: George W. Bush was the president of the US in 2007.
F.d.Roosevelt ( see warm springs movie)
33 of the 47 US vice-president never became President. See the related questions for more details.
"Marine One" ~ see related link below .
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If the president is unable to perform his duties, there is a procedure for giving his powers temporarily to the vice-president. See amendment 25 to the US constitution.
The Constitution states that the vice-president shall serve as president of the US Senate. The vice-president has no vote in Senate matters unless there is a tie vote, in which case, the vice-president casts the deciding vote. See Article I, Section 3, Clause 4 of the US Constitution.
Only a "natural born citizen" can hold the office of US President. See the related question below that gives the information from the Constitution that explains this requirement.
President of the US.