What are Greenbacks?

"The term Greenback was first used in referring to Demand Notes which were issued by the United States Government from 1861 – 1862. These demand notes were imprinted with a green United States government seal and it was the color of this seal that gave rise to the term Greenback. The first paper notes to be known as Greenbacks were a fiat currency issued during the American civil war. Being a fiat currency, Greenbacks were not backed by gold."
https://www.sunshineprofits.com/gold-silver/dictionary/greenback-gold/

"Greenbacks were paper currency (printed in green on the back) issued by the United States during the American Civil War. They were in two forms: Demand Notes, issued in 1861–1862, and United States Notes issued in 1862–1865. They were legal tender by law, but were not backed by gold or silver, only the credibility of the U.S. government"
https://en.wikipedia.org/wiki/Greenback_(1860s_money)

"Greenbacks got their name from their color. In the mid-1800s, the Continental Congress did not have taxing authority. The "greenback" was a negative term because they did not have secure financial backing authority and banks were reluctant to give customers the full value of the dollar."
https://www.investopedia.com/terms/g/greenback.asp

"Between 1862 and 1865, the U.S. government issued more than $450,000,000 in paper money not backed by gold (greenbacks) to help finance the Union cause in the American Civil War. After the war, fiscal conservatives demanded that the government retire the greenbacks, but farmers and others who wished to maintain high prices opposed that move. In 1868 the Democrats gave partial support to the Greenback movement by endorsing a plan that called for the redemption of certain war bonds by the issuance of new greenbacks."
https://www.britannica.com/event/Greenback-movement

"The objection to paper money was that it wasn't backed by precious metals, but rather by confidence in the issuing institution i.e. the federal government"
https://www.thoughtco.com/greenbacks-definition-1773325

"Pressure from business interests and creditors in the postwar period led to an effort to retire the greenbacks. These forces did not want to receive payments in cheap money and opposed any government policy that would lead to inflation. However the greenback prevailed…"
https://greenback.net/