USD is an abbreviation for United States Dollar, which is the official currency of the United States of America. Synonyms for USD include the greenback, buck, dollar bill, and the almighty dollar. Each of these terms represents the same currency but have different connotations and usage. Greenback refers to the color of the banknotes, while buck was derived from an earlier use of deer skins as currency in colonial America. Dollar bill is a common way to refer to the currency, and the almighty dollar is often used to emphasize its importance and influence worldwide. Regardless of the synonym used, USD remains a vital element of global trade and commerce.