Towards the end of the 19th century a number of major economic powers issued "crown"-type coins of special weight and value for use in commerce with countries in the Far East. The US created its own...
The last trade dollars were minted in 1885. They all carry the words TRADE DOLLAR on the back. Your coin is almost certainly a Morgan dollar. Please see the Related Question for more information.
First find out if its real. Take it to a qualified professional, Trade Dollars are about the most faked of all US coins. The date 1875 list 7 different coins for that year.