What is Dollar?

Legal Definition
Dollar (often represented by the dollar sign $) is the name of more than twenty currencies, including (ordered by population) those of the United States, Canada, Australia, Taiwan, Hong Kong, Singapore, New Zealand, Liberia, Jamaica and Namibia. The U.S. dollar is the official currency of East Timor, Ecuador, El Salvador, Federated States of Micronesia, Marshall Islands, Palau, the Caribbean Netherlands, and for banknotes, Panama. Generally, one dollar is divided into one hundred cents.
-- Wikipedia
Legal Definition
Money. A silver coin of the United States of the value of one hundred cents, or tenth part of an eagle.

2. It weighs four hundred and twelve and a half grains. Of one thousand parts, nine hundred are of pure silver and one hundred of alloy. Act of January 18, 1837, ss. 8 & 9, 4 Sharsw. Cont. of Story's L. U. S. 2523, 4; Wright, R. 162.

3. In all computations at the custom-house, the specie dollar of Sweden and Norway shall be estimated at one hundred and six cents. The specie dollar of Denmark, at one hundred and five cents. Act of May 22, 1846.
-- Bouviers Law Dictionary
Legal Definition
The unit employed in the United States in calculating money values. It ls coined both in gold and silver, and is of the vuiue of one hundred cents.
-- Black's Law Dictionary