Trade dollars are dollar coins that were minted in the US between 1873 in 1885. They were used within East Asia, and China in particular, to compete with the trade dollars from from other countries. The bulk of the coins that were struck in 1873 were shipped to to China. Eventually the coins made it back into the United States and were frowned upon by people here. They were actually trading for less than a dollar in the United States. As a result, they were demonetized in 1876 but they were re-monetized by the Coin Act of 1965.
It is interesting to note that United States Trade Dollars contains 90% silver and 10% copper, but they are not junk silver. Less than 36 million pieces were struck, and so there are few in existence today. Accordingly, they have more numismatic value than the value of their silver content.