A dollar was defined as a specific coin, containing 371.25 grains (troy) of fine silver.   This is equivalent to 0.77344 troy ounces of fine silver.

U.S. silver dollar coins minted prior to 1936 are said to have the same silver content as the Spanish milled dollar which was used for trade in early U.S. colonial times.  The silver content of the U.S. silver dollar remained constant from the 1794 until they were last minted in 1935.

The U.S. Treasury began minting a nominal one dollar silver coin again in 1986, known as a Silver Eagle.  However, it now contains one troy ounce of .999 fine silver.  This muddied the waters further, created additional confusion as to what a U.S. dollar really is, and what it is worth.

This article discusses history and politics regarding the U.S. dollar.  It is lengthy, but quite interesting.

https://www.constitution.org/mon/what_is_a_dollar.htm