The Collaborative International Dictionary of English v.0.48:

dollar \dol"lar\, n. [D. daalder, LG. dahler, G. thaler, an
   abbreviation of Joachimsthaler, i. e., a piece of money first
   coined, about the year 1518, in the valley (G. thal) of St.
   Joachim, in Bohemia. See Dale.]
   1.
      (a) A silver coin of the United States containing 371.25
          grains of silver and 41.25 grains of alloy, that is,
          having a total weight of 412.5 grains.
      (b) A gold coin of the United States containing 23.22
          grains of gold and 2.58 grains of alloy, that is,
          having a total weight of 25.8 grains, nine-tenths
          fine. It is no longer coined.
          [1913 Webster]

   Note: Previous to 1837 the silver dollar had a larger amount
         of alloy, but only the same amount of silver as now,
         the total weight being 416 grains. The gold dollar as a
         distinct coin was first made in 1849. The eagles, half
         eagles, and quarter eagles coined before 1834 contained
         24.75 grains of gold and 2.25 grains of alloy for each
         dollar.
         [1913 Webster]

   2. A coin of the same general weight and value as the United
      States silver dollar, though differing slightly in
      different countries, formerly current in Mexico, Canada,
      parts of South America, also in Spain, and several other
      European countries.
      [1913 Webster +PJC]

   3. The value of a dollar; the unit of currency, differing in
      value in different countries, commonly employed in the
      United States and a number of other countries, including
      Australia, Canada, Hong Kong, parts of the Carribbean,
      Liberia, and several others.
      [1913 Webster +PJC]

   Chop dollar. See under 9th Chop.

   Dollar fish (Zool.), a fish of the United States coast
      (Stromateus triacanthus), having a flat, roundish form
      and a bright silvery luster; -- called also butterfish,
      and Lafayette. See Butterfish.

   Trade dollar, a silver coin formerly made at the United
      States mint, intended for export, and not legal tender at
      home. It contained 378 grains of silver and 42 grains of
      alloy.