Currency is a unit of money that facilitates the purchase of goods or services. Each modern nation has one currency, such as the U.S. dollar in the United States, the pound sterling in The United Kingdom, and the Euro in several countries across Europe.
More specifically, currency is something (as coins, treasury notes, and banknotes) that is in circulation as a medium of exchange, paper money in circulation, or a common article for bartering. 
The invention of currency allowed civilization to advance beyond the barter system.
The denominations of currency produced in the United States are $1, $2, $5, $10, $20, $50, and $100. Larger denominations used be produced, including a $100,000 bill, featuring a portrait of Woodrow Wilson, that was produced at one time. In 1969, President Richard Nixon issued an executive order withdrawing bills larger than $100 from circulation "to make life harder for the mafia". 
The denominations of currency produced in the Euro Zone are 1c, 2c, 5c, 10c, 20c, 50c, 1€, 2€ coins, and 5€, 10€, 20€, 50€, 100€, 200€ and 500€ notes.
Australia converted to decimal currency on the 14th February 1966, issuing 1c, 2c, 5c, 10c, 20c, and 50c coins, and $1, $2, $10, and $20 notes. The $5 note was added the following year, and $50 and $100 notes were added in 1973 and 1984 respectively.
In 1983/4 and 1988 respectively, the $1 and $2 notes were replaced with coins. In 1991, the 1c and 2c coins were withdrawn from circulation. The 5c coin is now the smallest denomination of Australian currency.
Australia's colourful notes incorporate a range of security features, including embossing, raised print, a clear window, and fluorescent ink.
There has been a political debate over what the pros and cons of removing the 5c from the currency.
- The Australian currency Australian Government, Department of Foreign Affairs and Trade
- The Reserve Bank and Reform of the Currency: 1960–1988—Australia's First Decimal Currency Notes, Museum of Australian Currency Notes.
- The Reserve Bank and Reform of the Currency: 1960–1988—Inflation and the Note Issue, Museum of Australian Currency Notes.
- A New Era – Polymer Currency Notes: 1988 Onwards—A Complete Series of Polymer Notes, Museum of Australian Currency Notes.
- Security Features on Australia's Notes, Reserve Bank of Australia