The term dollar is, or has been, used by a number of nations to designate their primary currency unit.
The name originated as a result of the discovery of rich silver deposits in the Bohemian town of Joachimsthal, now known as Jáchymov in the Czech Republic in 1516. The local government minted this silver into large coins, known from the location of the mines as Joachimsthalers. The name was shortened to thalers, and corrupted in English speaking countries to dollars.
The name became applied to any silver coins approximately as large as the original thalers; in particular, the Spanish 8-reales piece, which circulated in large quantities in the English colonies that became the United States and Canada, since English silver coins were not allowed to be exported to the North American colonies and in those days coinage was supposed to consist of precious metal. These 8-reales pieces were called Spanish milled dollars, and became the de facto standard of exchange in much of the colonies, so the United States, on its independence, chose to adopt the dollar, equal to that silver coin, as its primary currency unit. Although the Canadian provinces remained English colonies until 1867 and politically united with the British even afterward, the Spanish milled dollar (and after U. S. independence, the United States dollar) was also the most common silver coin there, so that Canada adopted the dollar as well as its primary currency unit.
Influenced by the United States' example, many other nations adopted currency units termed dollars.