What is the symbol on the back of the US 1 dollar bill?

Answer 1

The symbol on the back of the U.S. dollar bill is the Great Seal of the United States. Here is the explanation provided by the Treasury Department:

The Great Seal of the United States on Paper Currency

The face (obverse) of the Great Seal first appeared on the back of the $20 Gold Certificate, Series 1905. In 1935, both the face and back (reverse) of the seal appeared for the first time on paper money on $1 Silver Certificates. Mandated by the First Continental Congress in 1776, the Great Seal took many years of work by multiple individuals and committees before final adoption in 1782. The Department of State is the official keeper of the seal. A description and explanation of both the obverse and reverse of the seal comes from the Department of State pamphlet "The Great Seal of the United States" (September 1996):

Obverse Side of the Great Seal: The most prominent feature is the American bald eagle supporting the shield, or escutcheon, which is composed of 13 red and white stripes, representing the original States, and a blue top which unites the shield and represents Congress. The motto E Pluribus Unum (Out of many, one), alludes to this union. The olive branch and 13 arrows denote the power of peace and war, which is exclusively vested in Congress. The constellation of stars denotes a new State taking its place and rank among other sovereign powers.

Reverse Side of the Great Seal: The pyramid signifies strength and duration: The eye over it and the motto Annuit Coeptis (He [God] has favored our undertakings) allude to the many interventions of Providence in favor of the American cause. The date underneath is that of the Declaration of Independence and the words under it, Novus Ordo Seclorum (A new order of the ages), signify the beginning of the new American era in 1776.