Thomas Jefferson was the third President of the United States of America. He was the first President that was inaugurated in the city of Washington DC. This is important because he helped plan and develop Washington DC.
Thomas Jefferson is the author of the Declaration of Independence. He is also responsible for the Louisiana Purchase, which allowed the United States to acquire all the French owned land in the continent. This act doubled the geographical size of the United States.
Thomas Jefferson is responsible for developing the decimal system, after his presidency. The American currency system is based on this development.